CBSE Class 12 Biology Principles of Inheritance and Variation Assignment Set A

Allele : Various or slightly different forms of a gene, having same position on chromosomes.

Phenotype : The observable or external characteristics of an organism

Genotype : The genetic constitution of an organism.

Monohybrid cross : A cross between two individuals of species, considering the inheritance of single pair of contrasting character e.g., a cross between pure tall (TT) and Dwarf (tt).

Dihybrid cross : A cross between two individuals of a species, considering the inheritance of two pairs of contrasting traits/characters e.g., a cross between Round and Yellow (RRYY) and wrinkled and green (rryy) pea seeds

Incomplete dominance : When one of the two alleles of a gene is incompletely dominant over the other allele.

Co-dominance : When two alleles of a gene are equally dominant and express themselves even when they are together.

Multiple allelism : When a gene exists in more than two allelic forms e.g., gene for blood group exist in three allelic forms, IA, IB and i.

Aneuploidy : The phenomenon of gain or loss of one or more chromosome(s), that results due to failure of separation of homologous pair of chromosomes during meiosis.

Trisomy : The condition in which a particular chromosome is present in three copies in a diploid cell/ nucleus.

Male heterogamety : When male produces two different types of gametes/ sperms e.g., In human beings X and Y.

Very Short Answer Questions Class 12 Biology Chapter 5 Principles of Inheritance and Variation

Question. Give an example of a chromosomal disorder caused due to non-disjunction of autosomes.
Answer. Down’s Syndrome.

Question. Name a human genetic disorder due to the following:
(i) An additional X-chromosome in a male
(ii) Deletion of one X-chromosome in a female
Answer. (i) Klinefelter’s Syndrome (ii) Turner’s Syndrome

Question. State what does aneuploidy lead to. 
Answer. Aneuploidy leads to individuals with abnormal number of chromosomes. Some disorder due to aneuploidy are Down’s Syndrome, Turner’s Syndrome, Klinefelter’s Syndrome.

Question. Name the type of cross that would help to find the genotype of a pea plant bearing violet flowers. 
Answer. Test cross.

Question. A garden pea plant produced axial white flowers. Another of the same species produced terminal violet flowers. Identify the dominant traits.
Answer. Axial, violet flower.

Question. Why, in a test cross, did Mendel cross a tall pea plant with a dwarf pea plant only?
Answer. To determine the genotype of the tall plant, whether it is homozygous dominant or heterozygous, as dwarfness is a recessive trait which is expressed only in homozygous condition and he was sure of genotype of dwarf plant.

Question. Name the event, during cell division cycle that results in the gain or loss of chromosome.s
Answer. Failure of segregation of chromosomes.

Question. If the frequency of a parental form is higher than 25% in a dihybrid test cross, what does that indicate about the two genes involved? 
Answer. It shows that the two genes are linked. 

Question. Write the types of sex determination mechanisms the following crosses show. Give an example of each type.
(i) Female XX with Male XO
(ii) Female ZW with Male ZZ 
Answer. (i) Male heterogamety, Grasshopper
(ii) Female heterogamety, Birds

Question. A haemophilic man marries a normal homozygous woman. What is the probability that their daughter will be haemophilic? 
Answer. 0% because only one X chromosome will carry the haemophilia gene. So, she will be a carrier.

Question. How many chromosomes do drones of honeybee possess? Name the type of cell division involved in the production of sperms by them. 
Answer. Drones possess 16 chromosomes. Mitosis is involved in the production of sperms.

Question. A male honeybee has 16 chromosomes whereas its female has 32 chromosomes. Give one reason. 
Answer. Male honeybee develops from unfertilised female gamete (Parthenogenesis) and thus has 16 chromosomes whereas female develops by fertilisation and thus has 32 chromosomes.

Question. State a difference between a gene and an allele. 
Answer. Gene contains information that is required to express a particular trait whereas alleles are alternating forms of a gene and are the code for a pair of contrasting traits for e.g., for plant height has two alleles – for tallness and dwarfness.

Question. Name the respective pattern of inheritance where F1 phenotype
(a) does not resemble either of the two parents and is in between the two.
(b) resembles only one of the two parents. 
Answer. (a) Incomplete dominance (b) Dominance

Question. Give an example of a human disorder that is caused due to a single gene mutation.
Answer. Sickle-cell anaemia.

Question. The egg of an animal contains 10 chromosomes, of which one is X-chromosome. How many autosomes would there be in the karyotype of this animal? 
Answer. There will be 9 pairs of autosomes in the karyotype of this animal.

Question. State the chromosomal defect in individuals with Turner’s syndrome. 
Answer. Monosomy of sex chromosome in females (XO condition).

Question. Name the stage of cell division where segregation of an independent pair of chromosomes occurs. 
Answer. Anaphase-I of Meiosis-I.

Question. How many type of gametes are produced by the individual with genotype AABBCCDD and AaBbCcDd? 
Answer. One type of gamete by individual (AABBCCDD) ABCD and sixteen (= 24 = 16) type of gametes by individual AaBbCcDd. 

Short Answer Questions Class 12 Biology Chapter 5 Principles of Inheritance and Variation

Question. What are the characteristic features of a true-breeding line? 
Answer. A true-breeding line for a trait is one that has undergone continuous self-pollination, showing a stability in the inheritance of the trait for several generations.

Question. In order to obtain the F1 generation, Mendel pollinated a true-breeding, say, tall plant with a true-breeding dwarf plant. But for getting the F2 generation, he simply self-pollinated the tall F1 plants. Why? 
Answer. All the F1 offsprings of the cross are heterozygous so allowing self-pollination is sufficient to raise F2 offspring. Also he intended to understand the inheritance of the selected trait over generations.

Question. In snapdragon, a cross between true-breeding red flowered (RR) plants and true-breeding white flowered (rr) plants showed a progeny of plants with all pink flowers.
(a) The appearance of pink flowers is not known as blending. Why?
(b) What is this phenomenon known as? 
Answer. (a) R (dominant allele red colour) is not completely dominant over r (recessive allele white colour). r maintains its originality and reappears in F2 generation. Therefore, it is not blending.
(b) Incomplete dominance.

Question. The phenotypic and genotypic ratio in F2 generation are same in a certain kind of inheritance.
Name an organism in which it occurs and mention the kind of inheritance involved. 
Answer. This kind of inheritance occurs in Mirabilis jalapa (4 O’clock plant) and the type of inheritance is called incomplete dominance.

Question. In a particular plant species, majority of the plants bear purple flowers. Very few plants bear white flowers. No intermediate colours are observed. If you are given a plant bearing purple flowers, how would you ascertain that it is a pure breed for that trait? Explain.
Answer. By test cross. Cross, purple flower plant with a (homozygous) recessive plant with white flowers, if all the flowers of the progeny are purple, the plant is homozygous dominant, i.e. pure breed.

Question. In snapdragon (Antirrhinum majus), a cross between varieties with red and white flowers produces all pink progeny. Explain how it is a case of incomplete dominance and not of blending inheritance.
Answer. In incomplete dominance, the genes of an allelomorphic pair are not expressed as dominant and recessive, but express themselves partially when present together in a hybrid and is an intermediate between the two genes. As a result an intermediate character is obtained. e.g., Two types of flowers occur in Mirabilis jalapa (4 o’ clock plant) and Antirrhinum majus (snapdragon/ dog flower). The red flower colour is due to gene RR, white flower colour is due to gene rr but pink flower colour appears in case of genotype Rr.It is not a case of blending inheritance because the parental characters reappear in the F2 generation without any modification. 

Question. What is a test cross? How can it decipher the heterozygosity of a plant? 
Answer. A cross to analyse whether genotype of dominant individual is homozygous or heterozygous is called test cross.
On crossing with a recessive parent, if 50% of progeny have dominant trait and 50% have recessive trait then the plant is said to he heterozygous.

Question. How does a test cross help to determine the genotype of an individual? 
Answer. In a test cross the individual of unknown genotype is crossed with the recessive parent. If all progenies are dominant, then the genotype exhibits homozygosity and if the progenies have a dominant to recessive ratio 1 : 1, then the genotype exhibits heterozygosity.

Question. With the help of one example, explain the phenomena of co-dominance and multiple allelism in human population. 
Answer. ABO blood group in human being is an example of multiple allelism and co-dominance. There are three alleles for the gene I, i.e., IA, IB, and i, thus, exhibiting multiple allelism. When IA and IB are present together the blood group is AB. Both A and B blood groups are expressed. This is called co-dominance.

Question. Explain pleiotropy with the help of an example.
Answer. Pleiotropy is the phenomenon in which a single gene exhibits multiple phenotypic expression.
The pleiotropic gene affects the metabolic pathways, resulting in different phenotypes. For example, phenylketonuria is caused by mutation in the gene coding the enzyme phenylalanine hydroxylase. It also leads to mental retardation & reduction in hair & skin pigmentation.

Question. Who proposed chromosomal theory of inheritance? Point out any two similarities in the behaviour of chromosomes and genes.
Answer. It was proposed by Sutton and Boveri.
Similarities:
(i) Both genes and chromosomes occur in pairs in a diploid cell (2n).
(ii) Both of them separate out during gametogenesis to enter into different gametes.
(iii) Paired condition is again restored by fusion of gametes. 

Question. In a dihybrid cross white eyed, yellow bodied female Drosophila crossed with red eyed, brown bodied male Drosophila produced in F2 generation, 1.3 per cent recombinants and 98.7 per cent progeny with parental type combinations. This observation of Morgan deviated from Mendelian F2 phenotypic dihybrid ratio. Explain, giving reasons, Morgan’s observations.
Answer. Morgan saw that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type. Morgan attributed this due to physical association or linkage of two genes and coined the term linkage to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non-parental gene combinations. %of recombinants depends on distance between genes. More is the distance more is % of recombinants and vice versa

Question. Linkage and crossing-over of genes are alternatives of each other. Justify with the help of an example. 
Answer. In Drosophila a yellow bodied white eyed female was crossed with brown bodied red eyed male.
The F1 progeny produced, when intercrossed, it was observed that the F2 phenotypic ratio of
Drosophila deviated significantly from Mendel’s 9 : 3 : 3 : 1. The genes for eye colour and body colour are closely located on the ‘X’ chromosome, showing linkage and therefore, these are inherited together. Recombinants were formed due to crossing over but at low percentage.

Question. How does the gene ‘I’ control ABO blood groups in humans? Write the effect the gene has on the structure of red blood cells. 
Answer. Gene ‘I’ has three different alleles IA, IB, i
(a) IA produces A type of sugar polymer on surface of RBC which results in A group
(b) IB produces B type of sugar polymer on surface of RBC which results in B group
(c) i produces no sugar which result in O blood group
The sugar polymers protrude from the surface of plasma membrane of RBCs which are characteristics of each blood group. 

Question. Human blood group is a good example of multiple allelism and co-dominance. Justify.
Answer. Multiple allelism: Generally in an individual or population, only two alleles of a trait govern the character, but in case of ABO blood group, three alleles IA, IB and i are found to govern blood group in human population. This is multiple allelism.
Co-dominance: Allele IA and IB when present in an individual, both being dominant express their own types of sugars or traits. Thus, exhibiting co-dominance.

Question. Give an example of a gene responsible for multiple phenotypic expressions. What are such genes called? State the cause that is responsible for such an effect. 
Answer. Gene causing phenylketonuria causes multiple phenotypic expressions. Such genes are called pleiotropic genes. This effect of multiple phenotypic expressions is caused because pleiotropic gene affects metabolic pathways, resulting in different phenotypes.

Question. Differentiate between male and female heterogamety. 
Answer. Differences between male heterogamety and female heterogamety

Question. Explain mechanism of sex determination in birds. 
Answer. In birds, females are heterogemetic and males are homogametic. Females have one Z sex chromosome and one W sex chromosome. Males have a pair of Z sex chromosome. If Z sperm fertilises Z ovum, a male offspring is produced, and if Z sperm fertilises W ovum a female offspring is produced.

Question.Explain the mechanism of sex determination in insects like Drosophila and grasshopper.
Answer. In grasshopper, the mechanism of sex determination is of the XO type. In females, the eggs bear a pair of X chromosomes along with the autosomes. Males contain only 1 X chromosome with autosomes. On the other hand, there are two types of sperms formed in males–one having a X chromosome and other without X chromosome. Hence, grasshopper shows male heterogamety.

Question. Differentiate between “ZZ” and “XY” type of sex-determination mechanisms.
Answer. ZZ type is seen in birds. The males are homogametic (ZZ) and females are heterogametic (ZY).
Sex is determined by the type of egg getting fertilised.
XY type is seen in human beings The males are heterogametic (XY) and females homogametic (XX). Sex is determined by the type of sperm fertilising the ovum.

Question. Why is pedigree analysis done in the study of human genetics? State the conclusions that can be drawn from it. 
Answer. Pedigree analysis is done because control crosses are not possible in case of humans beings.
This can be useful for analysis of traits, in several generations of a family, to trace pattern of inheritance to check whether the trait is dominant or recessive or sex-linked or not.