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- 5.1: Changes in Chromosome Number
- If something goes wrong during cell division, an entire chromosome may be lost and the cell will lack all of these genes. The causes behind these chromosome abnormalites and the consequences they have for the cell and the organism is the subject of this section.
- 5.2: Changes in Chromosome Structure
- If the chromosome is altered, but still retains the three critical features of a chromosome (centromeres, telomeres, and origin of replication), it will continue to be inherited during subsequent cell divisions, however the daughter cell may not retain all the genes. For example, if a segment of the chromosome has been lost, the cell may be missing some genes. The causes of chromosome structural abnormalites, which involves breaks in the DNA that makes up the chromosome.
Figure 5.8.1 Teenage Mutant Ninja Turtles Cosplay: Raphael and Michelangelo.
You probably recognize these costumed comic fans in Figure 5.8.1 as two of the four Teenage Mutant Ninja Turtles. Can a mutation really turn a reptile into an anthropomorphic superhero? Of course not — but mutations can often result in other drastic (but more realistic) changes in living things.
For Students & Teachers
For Teachers Only
Naturally occurring diversity among and between components within biological systems affects interactions with the environment.
Explain how chromosomal inheritance generates genetic variation in sexual reproduction.
Segregation, independent assortment of chromosomes, and fertilization result in genetic variation in populations.
The chromosomal basis of inheritance provides an understanding of the pattern of transmission of genes from parent to offspring.
Certain human genetic disorders can be attributed to the inheritance of a single affected or mutated allele or specific chromosomal changes, such as nondisjunction.
2nd PUC Biology Heredity and Variation One Mark Questions and Answers
Give the meaning of the term allele.
The alternate forms of a gene that occupies the same position on the homologous chromosomes are called alleles.
What is genetics?
Branch of biology which deals with the study of heredity and variations is called genetics.
The external appearance of an individual which has resulted due to the interaction between the genotype and the environment.
The total genetic constitution both expressed and suppressed of an organism is known as genotype.
Give the reason for Down’s syndrome.
Trisomy of 21 st chromosome.
Define test cross. What is its significance?
It is a cross made between the organism of unknown genotype with double recessive individual to determine the nature of unknown genotype (ie., homozygous/heterozygous).
If a diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
A diploid organism heterozygous for 4 loci, will produce 4 × 4 or 16 types of gametes.
Why is the blood group ‘O’ called as universal donor?
‘O’ blood group does not have any antigen and it is accepted by all the other blood group individuals. So it is regarded as universal donor,
Write the phenotypic ratio of Monohybrid cross.
3 : 1.
Mention the phenotypic ratio of a dihybrid cross.
9 : 3 : 3 : 1.
What are multiple alleles?
The phenomenon of occurrence of 3 or more alleles which have arisen as a result of mutation of normal genes and which occupy the same locus on homologous chromosomes are known as multiple alleles.
What genetic principle could be derived from a monohybrid cross?
Law of dominance.
Which one change is the cause of sickle-cell anaemia?
It is caused due to a point mutation at 6 th position in B-chain of hemoglobin in which glutamic acid is replaced by valine.
Name any one plant and its feature that shows the phenomena of incomplete dominance?
Mirabilis jalapa which, shows incomplete dominance in the colour of the flower.
2nd PUC Biology Heredity and Variation Two Marks Questions and Answers
What are multiple alleles? Give an example.
When a gene expresses itself in more than 2 allelic forms, then the alleles are called multiple alleles.
e.g: The gene ‘I’ controls the human blood group. It exist in 3 different alleleic forms i.e., I A , I B , and i, and is responsible for the four human blood groups viz, A, B, O and AB.
Give the chromosomal constitution and the resulting sex in each of the following syndrome.
(a) Turner’s syndrome
(b) Klinefelter’s syndrome
Turner’s syndrome – 22 pairs of autosomes and one X chromosome.
(44 + XO = 45 chromosome)
The resulting individual is a female.
Klinefelter ’s syndrome – 22 pairs of autosomes and XXY = [44 + XXY = 47 chromosomes]
The resulting individual is male with more female character.
Differentiate between the following:
(a) Dominance and Recessive genes
(b) Homozygous and Heterozygous
(c) Monohybrid and Dihybrid.
(a) Dominant gene – A gene which has the capacity to express in the hybrid condition.
Recessive gene – A gene which fails to express in the hybrid due to the presence of a dominant gene
(b) Homozygous – It is a situation of an individual in which a particular character is controlled by a pair of similar genes.
Heterozygous – It is a situation of an individual in which a particular character is controlled by a pair of dissimilar genes.
(c) Monohybrid : An hybridisation in which inheritance of only one character is followed from parents to offsprings, is known as a monohybrid cross.
Dihybrid : An hybridisation in which inheritance of two characters are followed from. one generation to another is called as dihybrid cross.
Explain the following terms with examples:
(b) Incomplete dominance.
(a) Co-dominance: It is a phenomenon where both the alleles in a heterozygous condition express themselves equally.
(b) It is a process where the dominant gene is incompletely dominant over the recessive gene and produces a phenotype which is intermediate to the parental type.
What is DNA polymorphism? Mention its significance.
Write any four characteristics of Down’s, syndrome.
Characteristics of Down’s syndrome include the following.
- Short stature, large head, small nose with flat nasal bridge.
- Open mouth with a protruding tongue.
- Eyes that slant upward with epicanthus fold.
- Stubby parted fingers and toes.
- Mental retardation.
Mention the possible blood groups of the progency whose mother is heterozygous ‘ for Group A and father is heterozygous for Group B.
A, B, AB and O.
List the antigens and antibodies of ‘A’ blood group and ‘O’ blood group?
Write any four characters of Turners syndrome?
- Broad chest and short body stature.
- Low hairline at the back of the neck.
- Shortened fourth and fifth fingers.
- Heart abnormalities.
- Hand and feet may be swollen at birth.
Write the chromosomal complement and two symptoms of Klinefelter’s syndrome. Chromosomal complement 44AA + XXY
- Gynoecomastia – Feminine characters like absence of facial hairs and presence of enlarged breast.
- Underdeveloped testis.
- Absence of spermatogenesis.
Differentiate between incomplete dominance and co-dominance.
|It is a phenomenon in which neither of the parental characters were completely expressed, instead a new intermediate character is expressed in the F1 generation.||It is a phenomenon where both the alleles in a heterozygous condition express themselves equally.|
Define linkage. Who discovered linkage in Drosophila?
Morgan and others observed that when the two genes in a dihybrid cross are located on the same chromosome, the proportion of parental gene combinations in the progeny was much higher than the non-parental or combinations (also called recombination) of genes.
Define Co-dominance? Give an example.
It is a phenomenon where both the alleles in a heterozygous condition express themselves equally.
- The blood group in humans is controlled by a single gene (I A ) with three alleles, I A , I B and i
- The gene I A and I B are dominant over the recessive allele i, whereas the allele I A and I B are co-dominant i.e. they are expressed together without being influenced by each other.
- Genetically, a person having blood group ‘A’ may be homozygous(I A I A ) or heterozygous (I A i)
- A person having blood group ‘B’ will be homozygous (I B I B ) or heterozygous (I B i)
- A person having blood group ‘AB’ will be (IA, IB), heterozygous co-dominant.
- A person having blood groups ‘O’ will be (ii), homozygous recessive
Define the terms autosomes and allosomes.
|are known as somatic chromosomes.||are known as sex chromosomes.|
|contain genes for somatic characters.||are responsible for sex determination.|
2nd PUC Biology Heredity and Variation Three Marks Questions and Answers
What is incomplete dominance? Describe with one example.
It is a process where the dominant gene is incompletely dominant over the recessive gene and produces a phenotype which is intermediate to the parental type.
In Mirabilis or 4° clock plant, incomplete dominance has been reported with respect to colour of the flower. In these plants some produce red flowers and some others produce white flowers. When a plant producing red flowers is crossed with a plant producing white flowers, in the F1 generation all plants produced pink flowers.
It is a heterozygous condition and the pink character is an intermediate or a blend between red and white This colour is produced because the red is being only partially dominant over white.
When F1 pink flower plants are self crossed to raise F2 progeny 3 types of plants are produced i.e, Red, Pink and White, in the ratio of 1 : 2 : 1 respectively.
By using appropriate symbols, the incomplete dominance can be represented as shown below. This cross is illustrated as indicated below by using appropriate letters.
Thus in this case, parental characters are expressed in homozygous conditions, and intermediate character in heterozygous condition.
Define the disorder of phenyketonuria.
- It is caused by a recessive mutant allele on chromosome 12.
- The affected individuals lack an enzyme that catalyses the conversion of the amino acid phenylalanine into tyrosine.
- Consequently phenylalanine is metabolised into phenly pyruvate and other derivatives.
- Accumulation of these chemicals in the brain results in mental retardation.
- These are also excreted in the urine as they are not absorbed by the kidney.
Mention the advantages of selecting the pea plant for experiment, by Mendel.
Mendel selected pea plant for his experiments because of the following reasons:
1. A large number of true breeding varieties with observable alternative forms for a trait were available.
2. Peas are normally self-pollinated. However, pea flowers can be readily cross pollinated (hybridised) if self pollination is prevented. This is achieved by removing the anthers (emasculation) before the pollen grains mature, and dusting the stigma of these flowers (female parent) with pollen from the desired plant (male parents).
3. Pea is an annual plant which gives results within a year.
4. A large number of seeds are produced by a pea plant, that helps in drawing correct conclusions.
5. Pea plants are easy to cultivate and do not require aftercare except at the time of pollination.
Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in F1 generation for a dihybrid cross?
(only two types of gametes will be produced from each parent as the two loci are linked.)
How is the sex determined in human beings?
In human beings, XY type of sex-determination is seen.
- The males have an X-chromosome and another small, but characteristically-shaped Y- chromosome, i.e., males have XY chromosomes along with other autosomes.
- The females have two X-chromosomes along with the other autosomes.
- It is a case of male heterogamety, where the males produce two types of sperms, 50% of sperms having one X-chromosome and the other 50% with one Y-chromosome.
- The females arc homogametic and produce all ova with one X-chromosome.
- Sex of an individual is decided at the time of fertilization by the type of sperm fertilizing the ovum.
It is as given below :
A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offsprings.
A person having blood group O has genptype (ii). He/she receives one allele from each of the parents for this trait. Therefore, his/her father with blood group A must have genotype (I A i) and mother with blood group B must have genotype (I B i).
The other possible genotype of their offsprings will be as follows :
By using Punnet square, schematically represent the dihybrid cross experiment conducted by Mendel using seed color and seed shape of pea as characters.
In a dihybrid cross, Mendel selected two pairs of contrasting characters. He selected the shape and colour of the seed as two characters, for his cross. He recognised round and wrinkled shape of the seeds as one pair of contrasting characters and yellow and green colour as another pair of contrasting characters.
He crossed a plant having Yellow and Round seeds with a plant having Green and Wrinkled seeds and got F1 generation. In the F1, he got all the plants producing Round and Yellow seeds. Thus he found that, yellow and round traits were dominant over green and wrinkled, respectively.
Later, he self-crossed the F1 hybrids and raised F2 generation. In this generation, he got four types of plants producing Yellow-Round, seed Yellow-Wrinkled seed, Green Round seed, and Green- Wrinkled seed. Thus he got two new characters-Yellow-Wrinkled seed and Green Round seed, in addition to the parental characters.
2nd PUC Biology Heredity and Variation Five Marks Questions and Answers
Explain the Law of Dominance using a monohybrid cross.
MONOHYBRID CROSS : Mendel selected two plants. One with tall and the other with dwarf to get F1 generation. To his surprise, he got all tall plants in the F1 generation.
Parents : Tall plant × Dwarf plant
F1 : → All tall plants
Thus the tall character appeared in the F1 generation but dwarf character did not appear. He self crossed the F1 plants and raised the F2 generation. Surprisingly he got both tall and dwarf plants in the ratio of 3:1 respectively. This ratio is called Phenotypic ratio. He repeated the experiment by taking other contrasting characters and got the same results.
Based on these results, he concluded that the tall character is dominant and the dwarf character is recessive. Then he used appropriate symbols to designate the dominant and recessive factors. He used capital letters for dominant factors and small letters of the same type to denote the recessive alleles. The monohybrid cross can be represented as below.
Number of plants of alleles involved -1 pair
Base Number – 4
Phenotypic ratio – 3 : 1 (3 tall: I dwarf
Genotypic ratio – 1 : 2 : 1
Phenotypic classes – Two (Tall and dwarf only)
Genotypic classes – Three
[1 homozygous tall, 2.heterozygous tall and 3. homozygous dwarf]
Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.
When a cross in made between a tall plant with yellow seeds (TtYy) and another tall plant with green seeds (Ttyy) , what proportions of phenotype in the offspring could be expected to be (a) tall and green (b) dwarf and green.
Write about chromosomal complement, cause and symptoms of Down’s syndrome.
Down’s syndrome is called as mongoloid idiocy or 21st trisomy. It is due to autosomal hyperploids i.e. trisomy in 21st pair where the chromosomal compliment is 45AA + xy for males and 45AA + xx for female.
- Flattened nose.
- Protruding tongue.
- Upward slanting eyes.
- Epicanthal fold (inner comer of eyes may have a rounded fold of skin).
- Hands are short and broad with short fingers.
- Hypotonia (Low muscular strength in infants).
- Short stature.
- Small or malformed ears.
- Mental dullness.
Explain Mendel’s dihybrid cross.
It is a cross between two plants differing in 2 pairs of contrasting characters.
Mendel selected shape of the seed and colour of the cotyledons as the two pairs of contrasting characters.
Round seed coat is dominant over wrinkled seed coat and yellow colour of cotyledon is dominant over the green colour.
Mendel crossed a pure breeding garden pea plant producing round and yellow seeds with a pure breeding garden pea plant producing wrinkled and green seeds.
Round seed coat and yellow cotyledons = 9
Wrinkled seed coat and yellow cotyledons = 3
Round seed coat and green cotyledons = 3
Wrinkled seed coat and green cotyledons = 1
Dihybrid phenotypic ratio = 9: 3 : 3 :1.
Genotypic ratio: 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1.
Thus in the F2 generation, he got Yellow Round, Yellow wrinkled, Green round and Green wrinkled in the ration of 9 : 3 : 3 : 1, respectively.
Explain sickle cell Anaemia.
This is an inborn error where the normal biconcave erythrocytes of an individuals are transformed into crescent or sickle shape. This is due to the production of defective S-haemoglobin. The production of normal Hb is controlled by the gene Hb whereas the other allele Hb controls the production of s-haemoglobin or sickle cell haemoglobin.
Therefore, this disease is not due to recessive gene but due to partially dominant pairs of alleles. When Hb occurs in homozygous condition (Hb A Hb A ) normal hemoglobin is produced in the body and the person will be healthy. But the presence of Hbs in homozygous (Hb s Hb s ) condition makes the person to suffer from chronic haemolytic anaemia.
It is due to the production of S- hemoglobin, which contains the amino acid valine instead of glutamic acid. This defective S-hemoglobin cannot transport oxygen. As a result, the RBC are deformed into sickle shape. Such RBCs are not able to move through the blood capillaries and therefore cause internal bleeding and pain. Such patients become anaemic and die.
The genetic code and the codon for glutamic acid is as follows:
Results: Glutamic acid is in the 6th position.
But under abnormal condition, instead of glutamic acid at the 6th position, valine, whose code and codon is as follows:
Result: Valine in the 6th position.
In heterozygous condition, both Hb A and Hb s genes are present: Such individuals produce both normal and S-haemoglobin This indicates that the genes HbA and Hbs are co-dominant such individuals suffer from periodic discomfort and anaemia at higher altitudes.
What are chromosomal disorders? Describe the following chromosomal disorders in human beings
a) Klinefelter’s syndrome
b) Turner’s syndrome.
a) Klinefelter Syndrome: This syndrome was reported by Harry Klinefelter. It is caused due to the presence of additional X in a genotype of XY. (Trisomy) The extrachromosomal number may go upto two or three in some instances XXXY and XXXXY.
Klinefelter syndrome is one of the most common causes of hypogonadism, affecting one in 500 births. In two thirds of cases, the chromosomal karyotype is 47, XXY, in males. Expression of the syndrome varies with t number of X-chromosomes and the degree of endocrinologic dysfunction. The patient’s history and physical examination provide enough information diagnose the condition in puberty, but most patients do not seek medical attention until adulthood, when problems such as impotency or infertility become an issue.
- Small penis
- Diminished pubic, axillary, and facial hair
- Enlarged breast tissue (called gynecomastia)
- Learning disabilities
- Simian crease (a single crease in the palm)
- Abnormal body proportions (long legs, short trunk)
- Small firm testicles
- Sexual dysfunction
- Tall stature
- Personality impairment.
Note : The severity of symptoms may vary.
Tests may include:
- Karyotyping showing 47 chromosomes with XXY
- Semen exam showing low sperm count
- Decreased serum testosterone level
- Increased serum luteinizing hormone and increased serum follicle stimulating hormone.
b) Turner’s syndrome: Harry Turner reported this syndrome. It is an allosomal numerical abnormality caused due to a loss of X chromosome in a female genotype of XX.
Turner syndrome is a genetic disorder affecting only females, in which the patient has one X – chromosome in some cells or has two X – chromosomes but one is damaged. Turner syndrome affects approximately 1 Out of every 2,500 female births worldwide. It embraces a broad spectrum of features, from major heart defects to minor cosmetic issues.
Some individuals with Turner syndrome may have only a few features, while others may have many. Almost all people with Turner syndrome have short stature and loss of ovarian function, but the severity of these problems varies considerably amongst individuals.
Module 5 / Inquiry Question 4
Genetic Variation created by crossing over
Crossing over is process involving the exchange of corresponding gene segments of non-sister chromatids between homologous chromosome pairs (double-stranded chromosome pairs for the most organisms).
This effectively creates creates new allele combinations, known as recombination.
It is important to stress that stating ‘new allele combinations being created as a result of crossing over’ is critical in HSC Biology as it is what appears on the marking criteria.
Below is a diagram illustrating the crossing over process between one pair of homologous chromosomes.
Notice that the crossing over occurs between non-sister chromatids, one from each double-stranded chromosome of the homologous pair. Each parent of the organism contribute one double-stranded chromosome.
For example, perhaps the blue one can be from the father (paternal chromosome) and the pink one can be from the mother (maternal chromosome).
Chiasma is the point where crossing over occur. Plural is chiasmata. There can be multiple chiasmata, usually the longer chromosome, the more points of crossing over. The chiasma the visible component of the homologous chromosomes during crossing over in Prophase I in meiosis I.
Recall from Week 2’s notes that crossing over does not occur in mitosis because the homologous chromosomes are NOT aligned side-by-side along the equator of the cell to allow overlapping segments of non-sister chromatids along the equator in the nuclear membrane.
Fun Fact: Crossing over can in fact occur between sister chromatids in one double-stranded chromosome (with requiring another double-stranded chromosome). However, it is ONLY crossing over between non-sister chromatids in a homologous pair where where new allele combinations are created.
Well, if you recall that sister chromatids are exact copies of each other, this means that the alleles on sister chromatids (coding for the any of the genes) are identical! Take a look at the diagram above. Each red dashed line highlights a locus on each chromatid.
Prior to crossing over, each locus (position) on each sister chromatid has the same allele.
For example, take the blue double-stranded chromosome, both of the sister chromatids have the ‘B’ allele that codes for black hair colour. Now, look at the other double-stranded chromosome (pink) in the homologous pair. At the locus of both sister chromatids that carries the gene that codes for eye colour, they both have the ‘b’ allele that codes for blue eye colour.
NOTE: In this example, ‘B’ is an allele means that it codes for black eye colour and ‘b’ is an allele codes for blue eye colour. The letters (or alleles to be exact) can code for different colours or genes depending on the question given to you on the day. For instance, in the exam, the question will tell you that what gene each letter codes for AND what the capital and lowercase of each letter would mean in terms of alleles.
By convention, capital letter represents a dominant allele and lowercase represents a recessive allele. We will talk about dominant and recessive alleles soon in this week’s notes.
Just to enhance your prior knowledge from Week 2: As you can see from the diagram shown above, the blue and red double-stranded chromosomes are called homologous pairs because , at the each locus (each locus shown by a red dashed line) of the chromosomes in the homologous pairs there are alleles that code for the same gene.
Fun Fact: The longer the distance genes are separated along each of the chromosome (chromatid to be precise), the greater the chance of crossing over. Generally, this will depend on the length of the chromosome. The greater the length, the more chance of crossing over and more chiasmata will exist during crossing over.
In the previous diagram, I have drew light green arrows on the homologous chromosomes after crossing over to indicate the alleles that belong to each of the four chromatids.
If I were to separate the four chromatids in the previous diagram on its own, the resulting allele combinations for the four chromatids would be as follows:
As you can see, there are two NEW allele combinations that has been created as a result of crossing over.
These new allele combinations are bHC and BhC which did not exist prior to crossing over.
Extra crossing over scenario that is BEYOND the HSC Biology Course
(You WILL NOT be asked about this in HSC Biology Exams)
Note that this diagram illustrating crossing over process, before and after, is slightly different to the previous diagrams. Can you spot the difference?
The difference is that the chiasma, i.e. the point of crossing over, is NOT located between the genes that codes for eye colour and height unlike in the previous diagram. As you can see in the products after crossing over, there is still new allele combinations formed. However, there is one consequence.
This type of crossing over usually is hard to detect (often undetected by normal means) compared to the previous crossing over scenarios. This means that the average number of crossing over detected would be lower than the true amount of crossing over that actually happened.
Scientists often perform crossing over experiments in attempt to determine the locus of chromosome (and thus DNA) that holds alleles that codes for a particular gene.
So, having undetected crossing over event that in fact actually occurred would yield inaccurate results.
This in fact relates to single nucleotide polymorphism which we will be covering as the last learning objective in this week’s notes.
Significance of crossing over
The process of crossing over creates new allele combinations adds diversity of the gene pool of the population as the four gametes formed in meiosis will not be identical but rather can have different an allele for each gene. Also, upon fertilisation, two random gametes with some variation in their alleles for certain genes will combine and produce an offspring with an unique allele combination to their parents. This therefore increases genetic variation in a population and can introduce new phenotypes that potentially* could be expressed.
Genetic variation in a population provides a pathway for evolution to occur. Without genetic variation, there will be no mechanisms for evolution. Genetic variation can be introduced into a population’s gene pool via:
Sexual reproduction (crossing over, independent assortment, random segregation and fertilisation) or
*The word ‘potentially’ is used because whether or not the allele will be expressed will depend on whether or not it is recessive and dominant which also depends on the complementary allele for the same gene.
Dominant and Recessive Alleles
Alleles are alternative (or different) versions of a gene that differ by their DNA sequence but codes for a protein that responsible for a same trait (e.g. hairy or hairless). An allele can be classified as dominant or recessive.
Dominant alleles are always expressed over recessive alleles if they are both present in the genotype for a gene of the organism.
By convention, dominant alleles are denoted with capital letters.
Recessive alleles are denoted with lower-case letters.
If the the alleles are coding for the same trait, the same letter is used. If not, different letters are used.
For example, in the diagrams shown previously, for the gene that codes for hair colour, there are two types of alleles, B and b.
The ‘B’ allele would be the dominant allele for hair colour and b would be the recessive allele for hair colour. In HSC Biology, the question will tell you what the hair colour for dominant allele as well as for the recessive allele for hair colour.
Some question requires you to determine what trait each allele expresses and whether or not they are dominant or recessive, in that case, the question will give you sufficient information to allow you to work it out. In that case, you will need to use Punnett Squares which we will learn this week. There will be these questions in this week’s homework set to allow you to practice. Give it a go. Solutions will be uploaded soon.
Anyhow, below are some rules of thumb about dominant and recessive allele combinations:
If the organism (parent and/or offspring) has two dominant allele for a particular gene, the dominant allele will be expressed.
If the organism (parent and/or offspring) has two alleles for a particular gene, one dominant and the other is recessive, the dominant allele will be expressed.
If the organism (parent and/or offspring) has two recessive alleles for a particular gene, the recessive allele will be expressed.
As illustrated in previous diagram, ‘Four different combinations of alleles after crossing over’, each chromatid begin segregating in Anaphase II and be completely segregated into a different gamete after Telophase II.
This occurs for all 23 pairs of homologous chromosome (one pair being sex chromosomes, XX or XY).
In the previous diagrams , we only drew two chromosomes (a homologous pair) that are crossing over just to satisfy the purpose of a simplistic illustration of the crossing over of chromosomes in a homologous pair.
As you may know now, when the gametes fuse together (egg and sperm except for fauna), a zygote is formed which has a diploid number of chromosomes. That is, the zygote has 46 chromosomes (23 homologous pairs).
So, in meiosis, the process of crossing over effectively creates the genetic variation in the gametes due to their difference in alleles for various genes (as we have mentioned earlier). Also, the random process of fertilisation between gametes (egg and sperm) also give rise to the genetic variation in the zygote whereby the two gamete have different alleles such that the zygote will have different allele combinations than its parents.
Due to meiosis, each gamete will have one allele that codes for a particular gene. Upon fertilisation where the female and male gametes fuse together to form a zygote, the zygote will then two alleles that code for each gene (e.g. gene for eye colour).
Therefore, the process of fertilisation creates new genotypes for the offspring that are, for the most part, different from each individual parent.
Note that if both parents the homozygous dominant for the same gene. Homozygous dominant just means that the individual has two same alleles (homozygous) and the alleles are dominant (capital letter). So, if both parents have homozygous dominant alleles for a trait, then the offspring will have genotype that is exactly the same as the parent (homozygous dominant too) though ONLY for that particular gene. It is unlikely that both parents will have EXACTLY the same genotype (allele combinations) for EVERY GENE . In that case, they be twins. LOL.
We will further explore and talk about homozygous dominant later when we look at Punnett Squares later in this week’s notes.
Also note, in reality, most traits (e.g. height) are governed by more than one gene.
Recall that independent assortment and random segregation both facilitate in increasing the genetic variation in offsprings of the new generations. Although they do not create new allele combinations, they do introduce variations in the way which alleles that specifies different genes on non-homologous chromosomes are assorted independently relative to each other (independent assortment) and eventually different chromatids that carries different alleles for certain genes can be segregated into different gametes (random segregation).
This means that these two independent assortment and random segregation introduce (genetic) variation into the alleles that each gamete inherit after Cytokinesis II in Telophase II.
The specifics in how independent assortment and random segregation increases the genetic variation in the offspring was discussed in Week 2’s note. If required, please revisit that section to refresh memory.
Recall that crossing over introduces new combinations of alleles that already exist in the chromatids by exchanging corresponding gene segments between non-sister chromatids.
Mutation, however, alters the allele’s identity because the DNA sequence is altered. This new DNA sequence can lead to an alternative expression of the gene (e.g. blue eye colour instead of black eye colour), hence new allele (different DNA sequence for same gene) is created.
In most cases, mutation only involves the modification of a DNA nucleotide, changing its nitrogenous base (point mutation).
In some more extreme cases, it can modify the DNA sequence of a portion of chromosome that involves one or more genes (chromosomal mutation).
We will explore point mutation and chromosomal mutation in Module 6 – Inquiry Question 1.
Anyhow, after learning about protein-synthesis in last week’s notes, where the creation of mRNA uses the DNA sequence of a gene as a template, it is clear that altering the nucleotide sequence of a gene will result in the specification of a different protein OR may modify the structure and function of the protein such that it will no longer functions as efficiently or completely not functional at all.
Suppose, you have an allele that codes for blue eye colour. This allele can be exchanged with another non-sister chromatid during crossing over.
However, mutation completely alters the identity of this allele originally coded for blue eye colour. This means that it may code for something slightly or completely different instead. Perhaps maybe, green eye colour.
Since alleles are altered, this would mean that through mechanisms of sexual reproduction and fertilisation, mutated allele will be inherited by gamete and possible give rise to offspring when fertilised. This would mean that a new allele will be introduced into the population. This would mean that there would be an increase in genetic variation in the population as allele frequency will increase.
Note that mutation on genes in autosomes and sex chromosomes will be passed onto offspring if ONLY the mutation occurs in the parents’ germ cell. The gametes derived from the parent germ cell then becomes an offspring when gamete is fertilised where the offspring can inherits the mutation. Most mutations passed onto offspring may not expressed as they could be recessive so offspring needs to inherit one mutated allele from each parent in order to express the mutated characteristic.
Whether or not the zygote will express the new allele due to mutation will depend on the zygote’s genotype (dominant or recessive for that gene) which we will explore very shortly when we deal with Punnett Squares.
If the mutated allele is expressed, it is important to consider whether or not the expressed trait that it codes for will become an improvement or hinderance to the organism’s daily activities.
Mutations are permanent changes to an organism’s DNA sequence.
Mutation and its implications will be discussed in greater detail in Module 6.
Mutation is considered the principal and original source in creating genetic variation in a population. This is because they provide the creation of new alleles by altering DNA sequence. The parent can be copy such mutated DNA via meiosis I which gametes and, when fertilised, the offspring can inherit such mutated DNA.
Not all of the four gametes must inherit such mutated DNA though.
Anyways, similar to crossing over, independent assortment, random segregation, we should now know that mutation also creates genetic variation in a population which provides a pathway for evolution to occur.
Without genetic variation, there will be no mechanisms for evolution. This ties nicely to support your responses when asking questions regarding ‘ensuring’ the continuity of species as per inquiry question one.
As genetic variability in a population increases, the species’ population ability to adapt to its environment over time also increases. It is through such adaptability of species in a population overtime whereby we witness evolution due to shifts in dominant or new characteristics in the species’ population.
Learning Objective #2 - Model the formation of new combinations of genotype produced during meiosis, including and interpreting examples of:
- Autosomal Inheritance
- Sex-Linkage Inheritance
- Co-dominance Inheritance
- Incomplete Dominance Inheritance
- Multiple alleles Inheritance
An autosome is a chromosome in an organism that is not a sex chromosome.
In humans, we have 22 pairs of autosomes and one pair of sex chromosomes. Genes are present in both autosomes and sex chromosomes.
The process of transferring genes (DNA) present in the parents’ autosomes to offspring is called autosomal inheritance.
The process of transferring genes (DNA) present in the parents’ sex chromosomes to offspring is called sex-linked inheritance or X-linked inheritance.
The genes that is present in autosomes and sex chromosomes, when passed to offsprings, exhibit different inheritance patterns or combinations of genotypes in terms of phenotype.
More specifically, the likelihood of an offspring exhibiting a recessive trait that is passed on autosomal inheritance is equal for both males and females. However, the likelihood of offspring exhibiting a recessive trait that is passed on via sex chromosomes inheritance is greater for males than in females.
Let’s have a look at why this is the case by exploring both modes of inheritance.
Recall that in humans, there are two alleles for a given gene where the alleles help determine the trait of organism (both the parents and offspring).
For illustration purposes, let’s use the gene that codes for eye colour. The gene that codes for eye colour will be present in both the female and male.
Just to get a clearer picture and as a recap, do recall that a gene is basically a segment of a DNA or a sequence of DNA nucleotides. Chromosomes are made up of DNA (wrapped around proteins called histones) for eukaryotes. Approximately 40% of a chromosome is made up of DNA and the other 60% is made up of proteins.
Now, let’s return to our example. So, each parent will have two alleles for its eye colour gene. Suppose that:
The mother has an allele that codes for blue eye colour and an allele that codes for black eye colour.
The father have both alleles that code for black eye colour.
Let’s also suppose that the allele that codes for black eye colour is denoted by ‘B’ and the blue eye colour is denoted by ‘b’.
As mentioned earlier, the dominant gene is denoted by a capital case of the letter and the recessive gene is denoted by the lower case version of the same letter. Therefore, in our case, black eye colour is a dominant gene whereas blue eye colour is a recessive gene.*
* In the exam you could either be told which is allele is dominant or recessive through the use of capital and lower case letters. If not, you will be told by words e.g. “Black eye colour is a dominant gene whereas blue eye colour is a recessive gene.” In that case, since no letter is given to you, you will need to chose a (same) letter to denote both alleles where the lower case letter represents the recessive allele and capital letter for the dominant allele.
Make sure you write down in words that letter you chose to use represent for that particular gene as well as what lower case and capital letter means in terms of recessive/dominant alleles and what characteristics they represent (e.g. black/blue eye colour).
Let’s have a look at how the alleles coding for eye colour can be passed on via the crossing over of the mother and father gametes. To do this, one method is to construct a Punnett Square.
Therefore, in our example, the mother will have the genotype, Bb, which is crossed over with the father with genotype, BB.
It does not matter which side (top or left of the square) you write the genotype for the male or female.
The order of the alleles also does not matter, e.g. (Bb or bB for the mother).
Each letter represents the allele for that particular gene (in this case gene for eye colour) that the parent inherited. Since each gamete only inherits one allele for each gene from each parent, you could think that each letter is essentially a gamete if you wish.
Recall that each parent has two alleles for the eye colour and it can pass on one of the two alleles to its offspring (B or b).
In the above example, the mother has B and b alleles for eye colour. So, she can pass on either the B or b allele to each of its gametes (egg cells) during meiosis. This is shown on the left of the Punnett Square.
For the father, he only two B alleles. So, he can only pass on B alleles to its gametes (sperm cells). This is shown on top of Punnett Square.
Don’t forget that each parent can produce many, many of gametes during his or her or its lifetime.
So, the Punnett Square therefore considers all the possible alleles which the parent have and could potentially pass onto its gamete and crosses these alleles together to depict the different allele combination for a particular gene which the offspring can inherit from both parents. The Punnett Square allows us to get four genotype combinations (allele combinations for a gene) as shown in the four boxes in the Punnett Square above. Some of the genotype combinations can be identical to each other as shown in the diagram.
When the mother and father crosses over, the resulting offspring will inherit one allele from each parent. There are four different possible genotype combination for the offspring’s eye colour. These are BB, Bb, BB and Bb. This means there are 50% chance that the offspring will have the genotype – BB. Also, there is 50% chance that the offspring will have genotype Bb.
Recall that the B allele is dominant over b allele (hence the capital letter). This means that although the offspring can inherit one of the two different genotype combinations, the resulting offspring only has one phenotype combination! That is, the offspring will have a 100% probability to have a black eye colour regardless of which genotype combination (BB or Bb) that its inherits from the parents.
In the exam, it is important for you to write in your response – either below or next to your Punett Square diagram, the following:
It is important to know that when both alleles for a gene of a parent are identical, the parent is called homozygous for that particular trait or gene. If the parent’s alleles for a gene are different, the parent are called heterozygous for that gene or trait. The same concept applies to offsprings.
Please DO NOT get the terms ‘homozygous’ and ‘homologous’ confused!
Homologous was used to refer to chromosomes pairs that have the carries genes at every corresponding locus. This is different to homozygous which refers to having different alleles of a particular trait.
If the organism contains one dominant allele and one recessive allele for a certain gene, it is called heterozygous dominant. For example, Bb.
If the organism contains two dominant alleles for a certain gene, it is called homozygous dominant. For example, BB.
Since there is only one case of recessive alleles, that is bb, it is just called recessive. Recessive is always homozygous.
Sex-Linkage or X-Linked Inheritance
Recall that earlier in this week’s note, we say that males are more likely to exhibit traits that are sex-linked inherited compared to females.
Suppose that you know of a disease where the disease exhibited more often in males and female such as colour-blindness, this would suggest that the alleles responsible for colour-blindness would be carried by sex chromosomes rather than autosomes as outlined earlier. We will have a look why by examining how a sex-linked disease such as colour-blindness can be passed on from parents to the offspring through allele inheritance.
Sex-linked traits are characteristics whereby the alleles responsible for them are only located on the X chromosome. This means that there is no allele responsible for the sex-linked trait on the Y chromosome. Only male gametes have Y sex chromosomes.
The alleles that are responsible for expression sex-linked traits are recessive.
This means that males only require one of the recessive allele for the sex-linked trait to be expressed as there is no second allele on the Y chromosome to allow the overriding of the recessive allele on the X chromosome in males.
Females, however, have two X sex chromosomes. In each chromosome, there is one allele that codes for the sex-linked trait (for example having colourblindness or normal colour vision). If one of the female’s X chromosome contains the recessive allele responsible for the colourblindness and the other X chromosome contains the dominant allele (normal colour vision), the female will not be colour-blind.
This is because the dominant allele from one of the X chromosome overridden the recessive allele that is responsible for expressing colour-blindness located on the other X chromosome.
As males only have an allele on its X chromosome and not on its Y chromosome, males are more likely to express sex-linked traits than females. This is because males only need one allele that codes for colour-blindness and the male will have the recessive sex-linked trait. For females, both recessive alleles must be inherited in order for a female to have the recessive sex-linked trait.
Notice in the above Punett Square diagram that we included genders of the parent, represented by their sex chromosomes, to show where alleles are carried on the sex chromosomes. Showing the sex chromosomes or gender is crucial to indicate to the marker that we are dealing with sex-linked inheritance.
This is important because the offspring’s gender (more specifically offsprings’ sex chromosomes) plays a role in determining whether or not the offspring will express the sex-linked trait such as colour-blindness.
Notice that in all scenarios when a male and female gamete (coming from father and mother respectively) is combined to form a zygote, there is equal chance of the offspring (zygote) being a male and female. This is consistent to reality.
Also notice that all males have a XY sex chromosome pair. All females have a XX sex chromosomes pair.
These sex chromosomes are within the male and female gametes of the parents.
Let’s explore at of the above scenarios, one at a time. Suppose that the allele is responsible for colour-blindness is denoted by a. The allele that is responsible for normal colour vision is denoted by A.
In scenario 1, as shown in the diagram, both the father’s and mother’s sex chromosomes do not have (i.e. do not carry) the allele responsible for colour-blindness. When the father and mother gametes crosses over, the offspring can either be female (XX) or male (XY) with equal chance. As the recessive colour-blindness allele is not present in either of the parents’ sex chromosomes, the offspring will not have genotype containing the colour-blindness allele. Therefore, the offspring that is born will not have colour-blindness regardless of it is male or female.
In scenario 2, as shown in the diagram, the father is affected by colour-blindness personally. That is, he is colour-blind. The mother is not colour-blind, nor she is a carrier of the allele responsible for colour-blindness. When the father and mother gametes combines to form the zygote, the offspring that develops can be either male or female with equal probability (as per usual). If the offspring is a female, then it will be a carrier of the recessive allele responsible for colour-blindness. That being said, all female offsprings will not be colour-blind themselves. Any female offspring produced will only be carriers of the allele responsible for colour-blindness.
In scenario 3, as shown in the diagram, the mother is a carrier of the recessive colour-blind allele whereas the father have an allele that codes for normal colour vision. When the two crosses over, if resulting offspring is male, the male offsprings will have a 50% chance to be colour-blind. All females offsprings will only be carriers of the recessive allele responsible for colour-blindness.
Scenario 4 (not included in diagram): If the father is colour-blind and the mother is a carrier, then 50% of all male offsprings will be colour-blind. For female offsprings, they have a 50% chance of suffering from colour-blindness. The other 50% for females will be carriers of the recessive allele responsible for colour-blindness.
Scenario 5 (not included in diagram): If the mother is affected then all male offsprings will be affected regardless of the father being colourblind or not.
So what is implication of Scenario 2?
Well, since the Y sex chromosomes (which is only present in males only) do not carry any alleles, if the female offsprings in scenario 2 crosses over with a male without colour-blindness, the resulting male offsprings in the new generation will have a 50% probability of suffering from colour-blindness! On the other hand, if the resulting offspring is female, they will have a 0% probability of suffering from colour-blindness.
What will happen if the father and mother both carry the colour-blindness allele and father and mother crosses over?
Well, 50% of the female offsprings will be colour-blind as the recessive allele is expressed. This is because 50% of the females will be carriers. There is also another 50% of female offsprings where both of the X sex chromosomes for the female will carry the recessive allele responsible for colour-blindness. If the female offspring inherits this combination of allele, this would mean that the female will be colour-blind.
If the offspring is a male, he will be colour-blind with 100% chance.
Conclusion: For traits that passed on via sex-linked inheritance, males have higher chance of exhibiting the sex-linked trait compared to females as the Y chromosome does not carry the relevant allele for the trait.
NOTE: It is important to remember that, at the end of the day, the allele combinations shown on the Punnett Square are all percentages used to indicate the probability of the offspring produced of getting a particular genotype (or allele combination). An offspring can inherit a genotype that has 25% probability and not a genotype that has 75% probability.
Co-dominance occurs when both alleles for a trait are expressed at the same time without blending of the trait that each allele specifies. This occurs when there is no recessive alleles for the trait.
An example of co-dominance is when a cow have both red and white colour patches on its skin. These cows are known as roan cows. Roan cows comes from the crossing over (mating) of red and white parent cows.
The offspring that have exhibit co-dominance must be heterozygous! They have different alleles for the same gene.
Let’s suppose you have a homozygous red female cow (RR) and a homozygous white male cow (WW). If these two cows are crossed, the cow offspring will 100% be roan (RW) as shown in the diagram below.
That is, the roan cow offspring will express both alleles where one codes for red coat colour and the other with white coat colour. This is because both alleles that codes for red and white colour patches are dominant, hence the term, co-dominance.
NOTE: Not all traits are co-dominant traits! Only in co-dominant traits where both alleles will be expressed as there is no dominant and recessive relationship between the alleles that govern the co-dominant trait.
CBSE Class 12 Biology –Chapter 5 Principles of Inheritance and Variation- Study Materials
- Genetics: Study of inheritance, heredity and variation of characters or Study of genes and chromosomes.
- Inheritance: Transmission of characters from parents to progeny. It is the basis of Heredity.
- Variation: Difference between parents and offspring.
- Character: A heritable feature among the parents & offspring. E.g. Eye colour.
- Trait: Variants of a character. E.g. Brown eye, Blue eye.
- Allele: Alternative forms of a gene. E.g. T (tall) and t (dwarf) are two alleles of a gene for the character height.
- Homozygous: The condition in which chromosome pair carries similar alleles of a gene. Also known as pure line (True breeding). E.g. TT, tt, YY, yy etc.
- Heterozygous: The condition in which chromosome pair carries dissimilar alleles of a gene. E.g. Tt, Yy etc.
- Dominant character: The character which is expressed in heterozygous condition. It indicates with capital letter.
- Recessive character: The character which is suppressed in heterozygous condition. It indicates with small letter.
- Phenotype: Physical expression of a character.
- Genotype: Genetic constitution of a character.
- Hybrid: An individual produced by the mating of genetically unlike parents.
- Punnett square: A graphical representation to calculate probability of all genotypes of offspring in a genetic cross.
MENDEL’S LAWS OF INHERITANCE
Gregor Mendel is the Father of genetics.
He conducted some hybridization experiments on garden peas (Pisum sativum) for 7 years (1856-1863).
Steps in making a cross (Deliberate mating) in pea:
- Selection of 2 pea plants with contrasting characters.
- Emasculation: Removal of anthers of one plant to avoid self-pollination. This is female parent.
- Pollination: Collection of pollen grains from the male parent and transferring to female parent.
- Collection & germination of seeds to produce offspring.
Mendel selected 7 pairs of true breeding pea varieties:
INHERITANCE OF ONE GENE
Monohybrid cross: A cross involving 2 plants differing in one character pair. E.g. Mendel crossed tall and dwarf pea plants to study the inheritance of one gene.
Monohybrid phenotypic ratio:
Monohybrid genotypic ratio:
1 Homozygous tall (TT)
2 Heterozygous tall (Tt)
1 Homozygous dwarf (tt)
Mendel made similar observations for other pairs of traits. He proposed that some factors were inherited from parent to offspring. Now it is called as genes.
Do not use T for tall and d for dwarf because it is difficult to remember whether T & d are alleles of same gene or not.
The F1 (Tt) when self-pollinated, produces gametes T and t in equal proportion. During fertilization, pollen grains of T have 50% chance to pollinate eggs of T & t. Also, pollen grains of t have 50% chance to pollinate eggs of T and t.
1/4th of the random fertilization leads to TT (¼ TT).
1/2 (2/4) of the random fertilization leads to Tt (½ Tt).
1/4th of the random fertilization leads to tt (¼ tt).
Backcross: Cross between a hybrid and its any parent.
Testcross: Crossing of an organism with dominant phenotype to a recessive individual. E.g.
Hence monohybrid test cross ratio= 1:1
Test cross is used to find out the unknown genotype of a character. E.g.
Mendel conducted test cross to determine the F2 genotype.
1. First Law (Law of Dominance)
- Characters are controlled by discrete units called factors.
- Factors occur in pairs.
- In a dissimilar pair of factors, one member of the pair dominates (dominant) the other (recessive).
2. Second Law (Law of Segregation)
“During gamete formation, the factors (alleles) of a character pair present in parents segregate from each other such that a gamete receives only one of the 2 factors”.
Homozygous parent produces similar gametes.
Heterozygous parent produces two kinds of gametes.
INHERITANCE OF TWO GENES
It is a cross between two parents differing in 2 pairs of contrasting characters.
E.g. Cross b/w pea plant with homozygous round shaped & yellow coloured seeds (RRYY) and wrinkled shaped & green coloured seeds (rryy).
On observing the F2, Mendel found that yellow and green colour segregated in a 3:1 ratio.
Round & wrinkled seed shape also segregated in a 3:1 ratio.
Dihybrid Phenotypic ratio:
9 Round yellow: 3 Round green: 3 Wrinkled yellow: 1 Wrinkled green = 9:3:3:1
The ratio of 9:3:3:1 can be derived as a combination series of 3 yellow: 1 green, with 3 round: 1 wrinkled.
Dihybrid genotypic ratio:
It is based on the results of dihybrid crosses.
It states that “When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters”.
Every gene contains information to express a particular trait.
In heterozygotes, there are 2 types of alleles:
- Unmodified (normal or functioning) allele: It is generally dominant and represents original phenotype.
- Modified allele: It is generally recessive.
E.g. Consider a gene that contains information for producing an enzyme. Normal allele of that gene produces a normal enzyme. Modified allele is responsible for production of
In the first case: The modified allele will produce the same phenotype like unmodified allele. Thus, modified allele is equivalent to unmodified allele.
In 2nd and 3rd cases: The phenotype will dependent only on the functioning of the unmodified allele. Thus the modified allele becomes recessive.
1. Incomplete Dominance
It is an inheritance in which heterozygous offspring shows intermediate character b/w two parental characteristics.
E.g. Flower colour in snapdragon (dog flower or Antirrhinum sp.) and Mirabilis jalapa (4’O clock plant).
Here, cross between homozygous red & white produces pink flowered plant. Thus phenotypic & genotypic ratios are same.
Phenotypic ratio= 1 Red: 2 Pink: 1 White (1:2:1)
Genotypic ratio= 1 (RR): 2 (Rr): 1(rr)
This means that R was not completely dominant over r.
Pea plants also show incomplete dominance in other traits.
It is the inheritance in which both alleles of a gene are expressed in a hybrid.
E.g. ABO blood grouping in human.
ABO blood groups are controlled by the gene I.
This gene controls the production of sugar polymers (antigens) that protrude from plasma membrane of RBC.
The gene I has three alleles IA, IB & i.
IA and IB produce a slightly different form of the sugar while allele i doesn’t produce any sugar.
Alleles from parent 1
Alleles from parent 2
Genotype of offspring
Blood types (phenotype)
When IA and IB are present together, they both express their own types of sugars. This is due to co-dominance.
It is the presence of more than two alleles of a gene to govern same character.
E.g. ABO blood grouping (3 alleles: IA, IB & i).
In an individual, only two alleles are present. Multiple alleles can be found only in a population.
4. Polygenic inheritance
It is the inheritance in which some traits are controlled by several genes (multiple genes).
E.g. human skin colour, human height etc.
It considers the influence of environment.
In a polygenic trait, the phenotype reflects the contribution of each allele, i.e., the effect of each allele is additive.
Assume that 3 genes A, B, C control human skin colour.
The dominant forms A, B & C responsible for dark skin colour and recessive forms a, b & c for light skin colour.
Genotype with all the dominant alleles (AABBCC) gives darkest skin colour.
Genotype with all the recessive alleles (aabbcc) gives lightest skin colour.
Therefore, genotype with 3 dominant alleles and 3 recessive alleles gives an intermediate skin colour.
Thus, number of each type of alleles determines the darkness or lightness of the skin.
Here, a single gene exhibits multiple phenotypic expressions. Such a gene is called pleiotropic gene.
In most cases, the mechanism of pleiotropy is the effect of a gene on metabolic pathways which contributes towards different phenotypes.
E.g. Starch synthesis in pea, sickle cell anaemia, phenylketonuria etc.
In Phenylketonuria & sickle cell anaemia, the mutant gene has many phenotypic effects. E.g. Phenylketonuria causes mental retardation, reduction in hair and skin pigmentation.
Starch synthesis in pea plant:
Starch is synthesized effectively by BB gene. Therefore, large starch grains are produced.
bb have lesser efficiency in starch synthesis and produce smaller starch grains.
Starch grain size also shows incomplete dominance.
CHROMOSOMAL THEORY OF INHERITANCE
Mendel’s work remained unrecognized till 1900 because,
- Communication was not easy.
- His mathematical approach was new and unacceptable.
- The concept of genes (factors) as stable and discrete units could not explain the continuous variation seen in nature.
- He could not give physical proof for the existence of factors.
In 1900, de Vries, Correns & von Tschermak independently rediscovered Mendel’s results.
Proposed by Walter Sutton & Theodore Boveri.
They said that pairing & separation of a pair of chromosomes lead to segregation of a pair of factors they carried.
Sutton united chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance. It states that,
- Chromosomes are vehicles of heredity.
- Two identical chromosomes form a homologous pair.
- Homologous pair segregates during gamete formation.
- Independent pairs segregate independently of each other.
Genes (factors) are present on chromosomes. Hence genes and chromosomes show similar behaviours.
Thomas Hunt Morgan proved chromosomal theory of inheritance using fruit flies (Drosophila melanogaster).
It is the suitable material for genetic study because,
- They can grow on simple synthetic medium.
- Short generation time (life cycle: 12-14 days).
- Breeding can be done throughout the year.
- Hundreds of progenies per mating.
- Male and female flies are easily distinguishable. E.g. Male is smaller than female.
- It has many types of hereditary variations that can be seen with low power microscopes.
Linkage is the physical association of two or more genes on a chromosome. They do not show independent assortment.
Recombination is the generation of non-parental gene combinations. It occurs due to independent assortment or crossing over.
Morgan carried out several dihybrid crosses in Drosophilato study sex-linked genes. E.g.
Cross 1: Yellow-bodied, white-eyed females X Brown-bodied, red-eyed males (wild type)
Cross 2: White-eyed, miniature winged X Red eyed, large winged (wild type)
Morgan intercrossed their F1 progeny. He found that
- The two genes did not segregate independently and the F2 ratio deviated from the 9:3:3:1 ratio.
- Genes were located on the X chromosome.
- When two genes were situated on the same chromosome, the proportion of parental gene combinations was much higher than the non-parental type. This is due to linkage.
- Genes of white eye & yellow body were very tightly linked and showed only 1.3% recombination.
- Genes of white eye & miniature wing were loosely linked and showed 37.2% recombination.
- Tightly linked genes show low recombination.Loosely linked genes show high recombination.
Alfred Sturtevant used the recombination frequency between gene pairs for measuring the distance between genes and ‘mapped’ their position on the chromosome.
Genetic maps are used as a starting point in the sequencing of genomes. E.g. Human Genome Project.
They include X & Y chromosomes.
Autosomes are chromosomes other than sex chromosomes.
Number of autosomes is same in males and females.
Henking (1891) studied spermatogenesis in some insects and observed that 50 % of sperm received a nuclear structure after spermatogenesis, and other 50 % sperm did not receive it. Henking called this structure as the X body (now it is called as X-chromosome).
- XX-XO mechanism: Here, male is heterogametic, i.e. XO (Gametes with X and gametes without X) and female is homogametic, i.e. XX (all gametes are with X-chromosomes). E.g. Many insects such as grasshopper.
- XX-XY mechanism: Male is heterogametic (X & Y) and female is homogametic (X only). E.g. Human & Drosophila.
- ZZ-ZW mechanism: Male is homogametic (ZZ) and female is heterogametic (Z & W). E.g. Birds.
XX-XO & XX-XY mechanisms show male heterogamety.
ZZ-ZW mechanism shows female heterogamety.
Human has 23 pairs of chromosomes (22 pairs of autosomes and 1 pair of sex chromosomes).
A pair of X-chromosomes (XX) is present in the female, whereas X and Y chromosomes are present in male.
During spermatogenesis, males produce 2 types of gametes: 50 % with X-chromosome and 50 % with Y-chromosome.
Females produce only ovum with an X-chromosome.
There is an equal probability of fertilization of the ovum with the sperm carrying either X or Y chromosome.
The sperm determines whether the offspring male or female.
It is based on the number of sets of chromosomes an individual receives.
Fertilised egg develops as a female (queen or worker).
An unfertilised egg develops as a male (drone). It is called parthenogenesis.
Therefore, the females are diploid (32 chromosomes) and males are haploid (16 chromosomes). This is called as haplodiploid sex determination system.
In this system, the males produce sperms by mitosis. They do not have father and thus cannot have sons, but have a grandfather and can have grandsons.
It is a sudden heritable change in DNA sequences resulting in changes in the genotype and the phenotype of an organism.
- Point mutation: The mutation due to change (substitution) in a single base pair of DNA. E.g. sickle cell anaemia.
- Frame-shift mutation: It is the deletion or insertion of base pairs resulting in the shifting of DNA sequences.
Loss (deletion) or gain (insertion/ duplication) of DNA segment cause Chromosomal abnormalities (aberrations).
Chromosomal aberrations are seen in cancer cells.
The agents which induce mutation are called mutagens. They include
- Physical mutagens: UV radiation, α, β, γ rays, X-ray etc.
- Chemical mutagens: Mustard gas, phenol, formalin etc.
In human, control crosses are not possible. So the study of family history about inheritance is used.
Such an analysis of genetic traits in several generations of a family is called pedigree analysis.
The representation or chart showing family history is called family tree (pedigree).
In human genetics, pedigree study is utilized to trace the inheritance of a specific trait, abnormality or disease.
The disorders due to change in genes or chromosomes.
2 types: Mendelian disorders & Chromosomal disorders.
It is caused by alteration or mutation in the single gene.
E.g. Haemophilia, Colour blindness, Sickle-cell anaemia, Phenylketonuria, Thalassemia, Cystic fibrosis etc.
The pattern of inheritance of Mendelian disorders can be traced in a family by the pedigree analysis.
Mendelian disorders may be dominant or recessive.
Pedigree analysis helps to understand whether the trait is dominant or recessive.
It is a sex linked (X-linked) recessive disease.
In this, a protein involved in the blood clotting is affected.
A simple cut results in non-stop bleeding.
The disease is controlled by 2 alleles, H & h.
Heterozygous female (carrier). She may transmit the disease to sons.
In females, haemophilia is very rare because it happens only when mother is at least carrier and father haemophilic (unviable in the later stage of life).
Queen Victoria was a carrier of hemophilia. So her family pedigree shows many haemophilic descendants.
It is a sex-linked (X-linked) recessive disorder due to defect in either red or green cone of eye. It results in failure to discriminate between red and green colour.
It is due to mutation in some genes in X chromosome.
It occurs in 8% of males and only about 0.4% of females. This is because the genes are X-linked.
Normal allele is dominant (C). Recessive allele (c) causes colour blindness.
The son of a heterozygous woman (carrier, XCXc) has a 50% chance of being colour blind.
A daughter will be colour blind only when her mother is at least a carrier and her father is colour blind (XcY).
This is an autosome linked recessive disease.
It can be transmitted from parents to the offspring when both the partners are carrier (heterozygous) for the gene.
The disease is controlled by a pair of allele, HbA and HbS.
- Homozygous dominant (HbAHbA): normal
- Heterozygous (HbAHbS): carrier sickle cell trait
- Homozygous recessive (HbSHbS): affected
The defect is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the β-globin chain of the haemoglobin (Hb).
This is due to the single base substitution at the sixth codon of the β-globin gene from GAG to GUG.
The mutant Hb molecule undergoes polymerization under low oxygen tension causing the change in shape of the RBC from biconcave disc to elongated sickle like structure.
An inborn error of metabolism.
Autosomal recessive disease.
It is due to mutation of a gene that codes for the enzyme phenyl alanine hydroxylase. This enzyme converts an amino acid phenylalanine into tyrosine.
The affected individual lacks this enzyme. As a result, phenylalanine accumulates and converts into phenyl pyruvic acid and other derivatives.
They accumulate in brain resulting in mental retardation. These are also excreted through urine because of poor absorption by kidney.
An autosome-linked recessive blood disease.
It is transmitted from unaffected carrier (heterozygous) parents to offspring.
It is due to mutation or deletion.
It results in reduced synthesis of α or β globin chains of haemoglobin. It forms abnormal haemoglobin and causes anaemia.
Based on the chain affected, thalassemia is 2 types:
- α Thalassemia: Here, production of α globin chain is affected. It is controlled by two closely linked genes HBA1 & HBA2 on chromosome 16 of each parent. Mutation or deletion of one or more of the four genes causes the disease. The more genes affected, the less α globin molecules produced.
- β Thalassemia: Here, production of β globin chain is affected. It is controlled by a single gene HBB on chromosome 11 of each parent. Mutation of one or both the genes causes the disease.
Thalassemia is a quantitative problem (synthesise very less globin molecules).
Sickle-cell anaemia is a qualitative problem (synthesise incorrectly functioning globin).
They are caused due to absence or excess or abnormal arrangement of one or more chromosomes.
- Aneuploidy: The gain or loss of chromosomes due to failure of segregation of chromatids during cell division.
- Polyploidy (Euploidy): It is an increase in a whole set of chromosomes due to failure of cytokinesis after telophase stage of cell division. This is very rare in human but often seen in plants.
It is the presence of an additional copy of chromosome number 21 (trisomy of 21).
Genetic constitution: 45 A + XX or 45 A + XY (i.e. 47 chromosomes).
- They are short statured with small round head.
- Broad flat face.
- Furrowed big tongue and partially open mouth.
- Many “loops” on finger tips.
- Broad palm with characteristic palm simian crease.
- Retarded physical, psychomotor & mental development.
- Congenital heart disease.
It is the presence of an additional copy of X-chromosome in male (trisomy).
Genetic constitution: 44 A + XXY (i.e. 47 chromosomes).
- Overall masculine development. However, the feminine development is also expressed. E.g. Development of breast (Gynaecomastia).
- Mentally retarded.
- Turner’s syndrome: This is the absence of one X chromosome in female (monosomy).
This is the absence of one X chromosome in female (monosomy).
Genetic constitution: 44 A + X0 (i.e. 45 chromosomes).
- Sterile, Ovaries are rudimentary.
- Lack of other secondary sexual characters.
- Mentally retarded.
CBSE Class 12 Biology Important Questions Chapter 5 – Principles of Inheritance and Variation
1 Mark Questions
Principles of Inheritance and Variation
1 Marks Questions
1. Give any two reasons for the selection of pea plants by Mendel for his experiments.
Ans.(i) Many varieties with contrasting forms of characters
(ii) Can easily be cross pollinated as well as self pollinated.
2. Name any one plant that shows the phenomenon of incomplete dominance during the inheritance of its flower colour.
Ans. Dog flower (Snapdragon or Antirrhinum sp.)
3. Name the base change and the amino acid change, responsible for sickle cell anaemia.
Ans. GAG changes as GUG, Glutamic acid is substituted by valine.
4. Name the disorder with the following chromosome complement.
(i) 22 pairs of autosomes + X X Y
(ii) 22 pairs of autosomes + 21st chromosome + XY.
Ans.(i) Klinefelter’s Syndrome (ii) Downs syndrome
5. A haemophilic man marries a normal homozygous woman. What is the probability that their daughter will be haemophilic?
Ans. Their daughter can never be haemophilic. (0%).
6. A test is performed to know whether the given plant is homozygous dominant or heterozygous. Name the test and phenotypic ratio of this test for a monohybrid cross.
Ans. Test cross 1 : 1.
7. Name the phenomena that occur when homologous chromosomes do not separate during meiosis.
Ans. Non – disjunction.
8. Name one trait each in humans & in drosophila whose genes are located on sex chromosome.
Ans. Humans – Colorblindness
Drosophila – Eye colour
9. What is meant by aneuploidy?
Ans. Aneuploidy is the phenomena of gain or loss of one or more chromosomes that results due to failure of separation of members of homologous pair of chromosomes during meioses.
10. What genetic principle could be derived from a monohybrid cross?
Ans. Law of dominance.
11. Which one change is the cause of sickle – cell anaemia ?
Ans. It is caused due to a point mutation at 6th position in B-chain of hemoglobin in which glutamic acid is replaced by valine.
12. What is a test cross?
Ans. It is a cross where offspring with dominant phenotype whose genotype is not known is crossed with an individual homozygous recessive for the trait.
13. What is mutagen? Give an example?
Ans. The physical or chemical agents that causes mutations are called mutagen eg x-rays, CNBr etc.
14. What was the total number of varieties of garden pea which Mendel had taken to start his experiment?
15. Name any one plant & its feature that shows the phenomena of incomplete dominance?
Ans. Antirrhium majus which shows incomplete dominance in flower colour.
2 Mark Questions
Principles of Inheritance and Variation
2 Marks Questions
1. Identify the sex of organism as male or female in which the sex chromosome are found as
(i) ZW in bird (ii) XY in Drosophila (iii) ZZ in birds. (iv) XO in grasshopper.
Ans. (i) Female (ii) Male (iii) Female (iv) Male
2. Mention two differences between Turner ’s syndrome and Klinefelter’s syndome.
Ans. Turners Syndrome : The individual is female and it has 45 chromosomes
i.e., one X chromosome is less.
Klinefelters Syndome : The individual is male and has 47 chromosomes
i.e., one extra X chromosome.
3. The human male never passes on the gene for haemophilia to his son. Why is it so?
Ans. The gene for haemophilia is present on X chromosome. A male has only one X chromosome which he receives from his mother and Y chromosome from father. The human male passes the X chromosome to his daughters but not to the male progeny (sons).
4. Mention four reasons why Drosophila was chosen by Morgan for his experiments in genetics.
Ans. (i) Very short life cycle (2-weeks)
(ii) Can be grown easily in laboratory
(iii) In single mating produce a large no. of flies.
(iv) Male and female show many hereditary variations
(v) It has only 4 pairs of chromosomes which are distinct in size and Shape.
5. Differentiate between point mutation and frameshift mutations.
Ans. Point Mutations : Arises due to change in a single base pair of DNA e.g., sickle cell anaemia. Frame shift mutations : Deletion or insertion/duplication/addition of one or two bases in DNA.
6. Give any two similarities between behavior of genes (Mendel’s factor) during inheritance & chromosomes during cell division.
Ans. (i) In diploid cells, the chromosomes are found in pairs just like that of mendelian factors.
(ii)During meiosis, the chromosomes of different homologous pairs are assorted independently into gametes at random showing parallelism with mendelian factors.
7. Which law of Mendel is universally accepted? State the law?
Ans. Mendel’s law of segregation is universally accepted It states that – “the two alleles of a gene remain separate & do not contaminate each other in F1 or the hybrid. At the time of gamete formation two alleles separate & pars into deferent gametes.
8. How will you find out whether a given plant is homozygous or heterozygous?
Ans. To test whether a plant is homozygous or heterozygous, test cross is performed in which individual is crossed with homozygous recessive for the trait. If plant is heterozygous, progeny of test cross consists of tall and dwarf plants in the ratio l:l
If plant is homozygous, progeny of test cross will have all tall plants
9. Why do sons of haemophilic father never suffer from this trait?
Ans. Since haemophilic is a sex – linked character, it shows criss – cross inheritance i-e from father to his daughter therefore son of haemopilic father is never haemophilic.
10. How is the child affected if it has grown from the zygote formed by an XX-egg fertilized by Y-carrying sperm? What do you call this abnormality?
Ans. If a child has grown from the zygote formed by XX-egg fertilized by Y-sperm, the child will suffer from klinefiter syndrome & will have XXY genotype. It is characterized by prominent feminine characters e.g. tall stature with feminised physique, Breast development pubic hair pattern, poor beard growth & sterility.
11. The map distance in certain organism between genes A & B is 4 units, between B & C is units, & between C & D is 8 units which one of these gene paves will show more recombination frequency? Give reason.
Ans. C& D will show maximum gene recombination because genes which are more closely linked, frequency of recombination is least & vice versa.
12. Give the chromosomal constitution & related sex in each of the following :-
i) Turner syndrome
ii) Klinefilter syndrome
ans. i) Turner syndrome – XO females containing 45 chromosomes & lacking one X-chr .
ii) Klinefilter syndrome XXY males containing 47chr, one extra X-chromosome in males.
13.What is pedigree Analysis? How is it useful?
Ans. The analysis of family history about inheritance of a particular trait in several generations of a family is called pedigree Analysis. It provides a strong tool which is utilized to trace inheritance of specific trait or abnormality or disease.
14. What are multiple alleles? Give an example?
Ans. The presence of more then two alleles of a trait is called multiple alleles e.g. in human beings four types of blood groups are recognized and there different alleles IA IB & IO of a gene determines the phenotype of four blood groups.
3 Mark Questions
Principles of Inheritance and Variation
3 Marks Questions
1. A woman with O blood group marries a man with AB blood group
(i) work out all the possible phenotypes and genotypes of the progeny.
(ii) Discuss the kind of dominance in the parents and the progeny in this case.
Ans. (i) Blood group AB has alleles as IA, IB and O group has ii which on cross gives the both blood groups A and B while the genotype of progeny will be IAi and IBi.
(ii) IA and IB are equally dominant (co-dominant). In multiple allelism, the gene I exists in 3 allelic forms, IA, IB and i.
2. Explain the cause of Klinefelter’s syndrome. Give any four symptoms shown by sufferer of this syndrome.
Ans. Cause : Presence of an extra chromosome in male i.e., XXY. Symptoms : Development of breast, Female type pubic hair pattern, poor beard growth, under developed testes and tall stature with Feminized physique.
3. In Mendels breeding experiment on garden pea, the offspring of F2 generation are obtained in the ratio of 25% pure yellow pod, 50% hybrid green pods and 25% green pods State (i) which pod colour is dominant (ii) The Phenotypes of the individuals of F1 generation. (iii) Workout the cross.
Ans. (i) Green pod colour is dominant
(ii) Green pod colour
Phenotypic ratio 3 : 1
Genotypic ratio 1 : 2 : 1
4. In Antirrhinum majus a plant with red flowers was crossed with a plant with white flowers. Work out all the possible genotypes & phenotypes of F1 & F2 generations comment on the pattern of inheritance in this case?
Ans. The inheritance of flower colour in snapdragon or Antirrhinum majus is an example of incomplete dominance. When a cross was made between a red flowered plant & a white flowered plant, the F1hybrid was pink i-e-an intermediate between red & white which means that both red & white are incompletely dominant. When F1 individuals was self – pollinated, the F2 generation consists of red, pink & white flower appears in ratio 1:2:1 respectively.
5. A red eyed male fruitfly is crossed with white eyed female fruitfly. Work out the possible genotype & phenotype of F1 & F2 generation. Comment on the pattern of inheritance in this cross?
Ans. When a red eyed is crossed with white eyed female fruitfly, offspring will have both white eyed male & red eyed female in 1:1 ration in F1 generation. In F2 generation, 50% females will be red – eyed & 50% will be white eyed, similarly, in males 50% will be red eyed & 50% will be white eyed. This result indicates that in sex-linked genes, males transmit their sex-linked characters to their grandson through their daughter such type of inheritance is called criss-cross inheritance –
6. A man with AB blood group marries a woman with O group blood.
(i) Work out all the possible phenotypes & genotypes of the progeny.
(ii) Discuss the kind of domination in parents & progeny in this case?
Ans. (i) Half the progeny will have blood group A with genotype IA IO & half the progeny will have blood group B with genotype IB IO.
(ii) IA & IB both the genes are dominant over IO gene hence progeny shows either blood group A or B while in parents since both the dominant genes are present together man will have blood group AB & this phenomena is called co-dominance.
7. In an cross made between a hybrid tall & red plant (TtRr) with dwarf & white flower (ttrr). What will be the genotype of plants in F1 generation?
8. How sex is determined in human brings?
Ans. In human beings, it was found that all the females bear a pairs of X-chromosome while males have one X-chr & also one Y-chr which is comparatively smaller in size.
Thus in a cross between male & female there is equal probability of males & females in progeny & sex is determined by presence of a Y-chr. if Y-chr is present it is male otherwise it is a female.
9. A smooth seeded & red – flowered pea plant (SsRr) is crossed with smooth seeded & white flowered pea plant (Ssrr). Determine the phenotypic & genotypic ratio in f1 progeny?
- Smooth seed & red flower =3
- Smooth seed & white flower =3
- Rough seed & red flower =1
- Rough seed & white flower =1
5 Marks Questions
Principles of Inheritance and Variation
5 Marks Questions
1. A dihybrid heterozygous round, yellow seeded garden pea (Pisum sativum) was crossed with a double recessive plant.
(i) What type of cross is this?
(ii) Work out the genotype and phenotype of the progeny.
(iii) What principle of Mendel is illustrated through the result of this cross?
Ans.(i) It is a dihybrid test cross
(iii) It illustrates the Principle of independent assortment.
2. In dogs, barking trait is dominant over silent trait & erect ears are dominant over drooping ears. What is the expected phenotypic ratio of offspring when dogs heterozygous for both the traits are crossed?
Ration :- Barking & erect = 9
Barking & drooping =3
Silent & erect = 3
Silent & drooping =1
Phenotypic ratio = 9 : 3 : 3 : 1
3. Differentiate between dominance, co-dominance & Incomplete dominance with one example each.
Ans. (i) Dominance :- When a cross is made between true – breeding tall pea plant & true – breeding dwarf pea plant, all the plants in F1 generation are tall this sows that tall character is dominant over dwarf
(ii) Co-dominance :- If the two equally dominant genes are present together, both of them will be equally expressed, this phenomena is called co-dominance eg alleles of blood group IA & IB ore dominant over IO but when both the alleles are present together, both of them will equally express & forms a phenotype AB.
(iii) In complete dominance :- When a cross is made between two characters of which none of them is completely dominant then an intermediate character develops in the progeny eg. when a cross is made between red flower & white flower in snapdragon flower an intermediate pink colour appears in the progeny
4. A dihybrid heterozygous tall & yellow pea plant was crossed with double recessive plant.
(i) What type of cross is this?
(ii) Work out the genotype & phenotype of progeny
(iii) What principle of Mendel is illustrated through result of this cross?
Ans. (i) Test cross.
(iii) Principle of Independent Assortment – Acc to which, in the inheritance of contrasting characters the factors of each pair of character segregate independently of the factors of the other pair of characters.
Sexual reproduction in humans requires that diploid individual cells produce haploid cells that can fuse during fertilization to form diploid offspring. The process that results in haploid cells is called meiosis. Meiosis is a series of events that arrange and separate chromosomes into daughter cells. During the interphase of meiosis, each chromosome is duplicated. In meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four haploid daughter cells, each with half the number of chromosomes as the parent cell. During meiosis, variation in the daughter nuclei is introduced because of random alignment in meiosis I. The cells that are produced by meiosis are genetically unique.
Meiosis and mitosis share similarities, but have distinct outcomes. Mitotic divisions are single nuclear divisions that produce daughter nuclei that are genetically identical and have the same number of chromosome sets as the original cell. Meiotic divisions are two nuclear divisions that produce four daughter nuclei that are genetically different and have one chromosome set rather than the two sets the parent cell had. The main differences between the processes occur in the first division of meiosis. The homologous chromosomes separate into different nuclei during meiosis I causing a reduction of ploidy level. The second division of meiosis is much more similar to a mitotic division.
Important similarities exist between spermatogenesis and oogenesis: during both processes a diploid cell is duplicated a number of times in mitosis to produce precursors that undergo two rounds of meiosis to produce haploid sperm and eggs. Important differences exist in the timing of these divisions and in the symmetry of divisions. In males, each spermatagonium produces four mature sperm, but in oogenesis each oogonium can produce only one fertilized egg.
Five Misconceptions in Genetics
Students may bring a variety of misconceptions with them when they enter a study of genetics. Watch your classroom for the 5 common misconceptions listed below. If you find any of them, just use the simple explanations𠅊lso provided below—to dispel your students’ incorrect notions.
- One set of alleles is responsible for determining each trait, and there are only 2 different alleles (dominant and recessive) for each gene. After learning about simple Mendelian inheritance and sex-linked traits, students often think that it is possible to model all traits so easily and predictably. In humans, at least 3 different genes are associated with eye color. Coat color in cats is controlled by at least 6 genes. Furthermore, the number of particular alleles inherited determines the expression of some characteristics for example, the number of alleles—that you inherit from each parent—that code for production of melanin may partially determine your hair color. Inheritance of more of the alleles may lead to darker hair, while inheritance of fewer may lead to lighter hair. For traits that show a Mendelian pattern of inheritance, students often assume that there are only 2 possible alleles for a trait. This is true in some cases, but in many cases, there are more alleles for a trait. In cat-coat-color genetics, 3 different alleles of 1 gene determine the position of pigmentation on the body.
- Your genes determine all of your characteristics, and cloned organisms are exact copies of the original. While genes play a huge role in how an organism develops, environmental factors also play a role. Epigenetics is the study of heritable changes that occur without changes in the genome. The gene expression in identical mice has shown changes from factors such as diet and exposure to toxins. Further studies with identical twins have suggested that these changes can accumulate over the life of the organism. The cloning of Rainbow, a domestic cat, demonstrated 1 striking example of epigenetics. Rainbow’s coat showed calico coloration, while the coat of the clone, named Copycat, is a tabby pattern. Because Copycat and Rainbow had identical genomes, the differences must be due to epigenetic factors.
- All mutations are harmful. A mutation is a change in the genetic code of an organism. Many mutations are harmful and cause the organism not to develop properly. However, many mutations are silent and some prove beneficial. In the case of a silent mutation, the change in the genome does not change the production of the amino acid sequence and subsequent protein (remember that multiple codons may code for the same amino acid, so a change in 1 nucleotide does not necessarily change the gene product). If an organism does live with a mutation, then often the environment will determine whether the mutation is beneficial or harmful. Production of 1 protein vs. another may confer a characteristic such as a difference in coloration or in the ability to digest a resource (e.g., the ability to digest lactose or maltose instead of sucrose). The phenotypic outcome may be selected, for or against, depending on environmental factors.
- A dominant trait is the most likely to be found in the population. The term 𠇍ominant allele” sometimes conveys to students the impression that the allele is the one that exists in the greatest proportion in a population however, 𠇍ominant” refers only to the allele’s expression over another allele. Human genetics includes examples of dominant traits that do not affect the majority of the population. In fact, achondroplasia, a type of dwarfism caused by the presence of a dominant allele, is found in fewer than 1 in 10,000 live births. Huntington’s disease, a degenerative disease caused by the presence of a dominant allele, occurs at a rate of about 3 to 7 cases per 100,000 people of European descent.
- Genetics terms are often confused. Many students understand the basic ideas of genetics but need more familiarity with the terms. For example, students often struggle with the difference between a chromosome, a gene, and an allele. Chromosomes are organized structures containing proteins and a single coiled strand of DNA chromosomes are visible with a microscope only during parts of the cell cycle. Genes are units of heredity—specific sequences of DNA or RNA that create proteins with particular functions in an organism. Alleles are variants of a gene. Making sure that students have a strong foundation in the terminology can greatly improve their understanding of genetics and prevent misconceptions.
Dispelling these 5 misconceptions will help students better understand genetics information and activities that you plan for both the classroom and the lab. They will also realize there are many influences on the way living things develop genetically over time.
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5.2 Review Questions
- What are chromosomes and genes? How are the two related?
- Describe human chromosomes and genes.
- Explain the difference between autosomes and sex chromosomes.
- What are linked genes, and what does a linkage map show?
- Explain why females are considered the default sex in humans.
- Explain the relationship between genes and alleles.
- Most males and females have two sex chromosomes. Why do only females have Barr bodies?
5: Chromosome variation - Biology
5.1.2 Meiosis and Variation
a) describe, with the aid of diagrams and photographs, the behaviour of chromosomes during meiosis, and the associated behaviour of the nuclear envelope, cell membrane and centrioles. (Names of the main stages are expected, but not the subdivisions of prophase)
- Best way to remember mitosis PMAT x2
- But before that interphase occurs
- In interphase the DNA replicates so that instead of the 2 chromosomes in the cell, we will now have 4.
- Prophase I
- The Chromatin condenses and undergoes supercoiling
- The chromosomes come together in the homologous pairs to form a bivalent
- Non sister pairs wrap around each other at points called the charismata
- Crossing over (exchange of genes) occurs at these points
- The nucleolus disappears and the nuclear envelope disintegrates
- Spindles of protein microtubules form
- Bivalent pairs line up vertically on the equator
- Spindles attach at the centromeres
- Bivalent pairs are arranged randomly (independent assortment)
- Spindles pull the bivalent apart to opposite parts of the pole
- Two nuclear envelopes form and the cell devices via cytokinesis.
- In animals there is a Interphase
- In Plants there in no Telophase
- Prophase II
- If there is a nuclear envelope then it brakes down
- the nucleolus disappears and the chromatids condense
- Chromosomes arrange themselves horizontally across the equator
- Spindles form
- The chromatics are randomly assorted
- Spindles pull apart the chromosome
- Nucleolus form at each pole
- Cells divide via cytokinesis
- In animals 4 haploid cells form
- In plants a tetrad (2x2 area) forms
b) explain the terms allele, locus, phenotype, genotype, dominant, codominant and recessive
- allele - An alternative version of a gene
- locus - A specific position on a chromosome occupied by a specific gene
- phenotype - Observable characteristics of an organism
- genotype - The alleles present within the cells of an individual for a particular trait/characteristic
- dominant - A characteristic that is always expressed in the phenotype even those in heterozygous genotypes
- codominant - A characteristic where both alleles contribute to the phenotype
- recessive - A characteristics that only expressed in the phenotype when there is no dominant allele
c) explain the terms linkage and crossing-over
- linkage - Gene's for different characteristics that are at different loci on the same chromosome are linked
- crossing over - Where non sister chromatids exchange alleles during prophase 1 of meiosis
d) explain how meiosis and fertilisation can lead to variation through the independent assortment of alleles
- Meiosis increases genetic variation by:
- Crossing over during Prophase I
- Genetic reassortment due to random assortment and segregation of chromosomes in Meiosis I
- Genetic reassortment due to random assortment and segregation of chromatids in Meiosis II
- Random Mutations
- Fertilisation causes variation by combining two genetically un-identical pairs
e) use genetic diagrams to solve problems involving sex linkage and codominance
f) describe the interactions between loci (epistasis). (Production of genetic diagrams is not required)
g) predict phenotypic ratios in problems involving epistasis
h) use the chi-squared (χ2) test to test the significance of the difference between observed and expected results. (The formula for the chi-squared test will be provided)
i) describe the differences between continuous and discontinuous variation
- Describes the quantitative differences in between phenotypes.
- Where phenotypic differences show a range of variation that can't be put into categories
- Describes qualitative differences between phenotypes.
- The variation produced can be easily put into categories.
j) explain the basis of continuous and discontinuous variation by reference to the number of genes which influence the variation
- controlled by two or more genes
- each gene provides an adaptive component to the phenotype
- different alleles at each gene locus have a small effect on the phenotype
- a large number of different genes may have a combined effect on the phenotype. These are called polygenes and their effect is called polygenic. The genes are unlinked (on different chromosomes)
- different alleles at the same gene locus have a large effect on the phenotype
- different gene loci can have a large effect on the phenotype
- examples include codominance, dominance and recessive patterns of inheritance
k) explain that both genotype and environment contribute to phenotypic variation. (No calculations of heritability will be expected)
- Genes can give the potential for the characteristic to occur however the environment largely determines the outcome because if the animal say doesn't get enough nutrients to grow then they won't grow regardless of the what the gene says.
- This effects polygenes more than mono genes.
l) explain why variation is essential in selection
- Genetic variation is a requirement of selection because in order for any type of selection to work there must be organisms with different phenotypes which can then be selected and promoted.
m) use the Hardy–Weinberg principle to calculate allele frequencies in populations (HSW1)
- (p + q)² = p² + 2pq + q² = 1
- q = 1 - p
- The population is very large
- The mating within the population is random
- There is no selective advantage for any genotype
- There is no mutation, migration or genetic drift
n) explain, with examples, how environmental factors can act as stabilising or evolutionary forces of natural selection
- If the environment remains stable then so does the selection pressure hence any variation won't likely give an advantage to the organism and may even be destructive for the organism.
- This prevents those genes from being passed on.
- However, of the environment changes and produces a new selection pressure then the organisms best suited to the environment will be able to live long enough to reproduce to pass on their genes.
o) explain how genetic drift can cause large changes in small populations
- Reduces genetic variation and may reduce the ability of an organism to survive change because there are so few copies of alleles
p) explain the role of isolating mechanisms in the evolution of new species, with reference to ecological (geographic), seasonal (temporal) and reproductive mechanisms
- Geographic barriers and seasonal barriers will cause allopatric speciation
- reproductive mechanism that are incompatible will cause sympatric speciation
q) explain the significance of the various concepts of the species, with reference to the biological species concept and the phylogenetic (cladistic/evolutionary) species concept (HSW1)
- Biological species concept
- A group of similar organisms that can breed together to produce offspring that are fertile.
- A group of organisms that have the same morphology, physiology, embryology, behaviour and occupy the same ecological niche.
r) compare and contrast natural selection and artificial selection
- In natural selection those who are best adapted will survive long enough to pass their genes on to their offspring.
- The environment is doing the selecting
s) describe how artificial selection has been used to produce the modern dairy cow and to produce bread wheat (Triticum aestivum) (HSW6a, 6b).
Watch the video: What is a Chromosome? (May 2022).