CBSE Class 12 Biology Principles Of Inheritance And Variation Worksheet Set E

Very Short Answer Questions

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: 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: Discuss is the genetic basis of wrinkled phenotype of pea seeds. 
Answer. Wrinkled seed shape is a recessive trait. It expresses only under homozygous condition of alleles.

Question: When a tall pea plant was self-pollinated, one-fourth of the progeny were dwarf. Give the genotype of the parent and dwarf progenies.
Answer. Genotype of parent is Tt and the genotype of dwarf progenies is tt.

Question: How many kinds of phenotypes would you expect in F2 generation in a monohybrid cross?
Answer. Two (e.g., Tall and dwarf).

Question: A garden pea plant (A) produced inflated yellow pod, and another plant (B) of the same species produced constricted green pods. Identify the dominant traits.
Answer. Inflated green pod is the dominant trait.

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

Question: A garden pea plant produced round green seeds. Another of the same species produced wrinkled yellow seeds. Identify the dominant traits.
Answer. Round, yellow seed are the dominant traits.

Short Answer Questions

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: A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?
Answer. Here, we apply the formula 2n where n = number of loci.
The organism is heterozygous for 4 loci,
n = 4
So, 2n = 24 = 2 × 2 ×2 × 2 = 16
∴ The organism will produce 16 types of gametes.

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.

QuestionQ. 10. 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: With the help of a Punnett square, find the percentage of homozygous talls in a F2 population involving a true breeding tall and a true breeding dwarf pea plant.
Answer.
Percentage of homozygous tall = 1/4×100 = 25%

Question: Mention the advantages of selecting pea plant for experiment by Mendel.
Answer. - Mendel selected garden pea as his experimental material because of the following reasons:
(i) It is an annual plant with a short life-cycle. So, several generations can be studied within a short period.
(ii) It has perfect bisexual flowers containing both male and female parts.
(iii) The flowers are predominantly self-pollinating. It is easy to get pure line for several generations.
(iv) It is easy to cross-pollinate them because pollens from one plant can be introduced to the stigma of another plant by removing the anthers.
(v) Pea plant produces a large number of seeds in one generation.
(vi) Pea plants could easily be raised, maintained and handled.
(vii) A number of easily detectable contrasting characters

Question: Define and design a test cross.
Answer. 

- It is a method devised by Mendel to determine the genotype of an organism.
- In this cross, the organism with dominant phenotype (but unknown genotype) is crossed with the recessive individual. 
- In a monohybrid cross between violet colour flower (W) and white colour flower (w), the F1 hybrid was violet colour flower. The test crosses are:

Question: Differentiate between the following:
(a) Dominance and recessiveness
(b) Homozygous and heterozygous
(c) Monohybrid and dihybrid
Answer. (a) Table 5.3: Differences between dominance and recessiveness

S.No. Dominance Recessive
(i) The phenomenon where one allele expresses itself even in the presence of other allele. The phenomenon where an allele expresses itself in the absence of its dominant allele but remains masked in its presence.
(ii) Dominant allele forms a complete functional enzyme due to which complete polypeptide is formed to express completely. Recessive allele forms incomplete or defective, or non-functional polypeptide enzyme, due to which non-functional polypeptide is formed and fails to express completely.

(b) Differences between homozygous and heterozygous

S.No. Homozygous Heterozygous
(i) When both alleles of a gene are similar, then the individual is called homozygous. When both alleles of a gene are dissimilar, then the individual is called heterozygous.
(ii) The genotype is expressed as TT or tt, i.e., they contain either both dominant alleles or both recessive alleles. The genotype is expressed as Tt, i.e., they contain one dominant allele and one recessive allele.
(iii) They are true breeding, leading to pure lines. They are not true breeding.
(iv) The gametes produced by them are similar in genotype. The gametes produced by them are of two types, one with dominant allele and other with recessive allele.

(c) Differences between monohybrid and dihybrid

S.No. Monohybrid Dihybrid
(i) It is the cross between two individuals considering a single contrasting trait or character at a time. It is the cross between two individuals taking two contrasting traits or characters at a time.
(ii) It helps to study the inheritance of a pair of allele. It helps to study the inheritance of two pairs of allele.
(iii) The phenotypic ratio in F2 generation is 3 : 1. The phenotypic ratio in F2 generation is 9 : 3 : 3 : 1.
(iv) The genotypic ratio in F2 generation is 1 : 2 : 1. The genotypic ratio in F2 generation is 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1. 

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. (Any two)

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.

Long Answer Questions

Question:
(a) How do human males with ‘XXY’ abnormality suffer?
Answer. (a) The XXY individual suffers from Klinefelter’s syndrome.

Question: A pea plant with purple flowers was crossed with white flowers producing 50 plants with only purple flowers. On selfing, these plants produced 482 plants with purple flowers and 162 with white flowers. What genetic mechanism accounts for these results? Explain.
Answer. The gene for purple flowers is dominant over that of white flowers. So, when two pure varieties are crossed, the F1 generation has only purple flowers and on selfing, the flowers are produced in a 3 : 1 ratio in F2 generation.

This result is obtained due to segregation of the alleles at the time of gametogenesis. The alleles remain together in a zygote but during gamete formation, they segregate such that the gametes carry only one allele.

Question: (a) You are given tall pea plants with yellow seeds whose genotypes are unknown. How would you find the genotype of these plants? Explain with the help of cross.
(b) Identify a, b and c in the table given below: (image 183)
Answer. (a) Test cross will be performed to know the genotype of these plants. (image 184)

If all the plants of F1 generation are tall with yellow seeds, then the phenotype of the parent is homozygous dominant (case i). If the plants in F1 generation are in the ratio of 1 : 1 : 1 : 1, then the parent plant is heterozygous dominant.
(b) a–Both the forms of a trait are equally expressed in F1 generation.
b–Dominance.
c– Phenotypic expression of F1 generation is somewhat intermediate between the two parental forms of a trait.

Question: (a) Write the conclusions Mendel arrived, at on dominance of traits on the basis of monohybrid crosses that he carried out in pea plants.
(b) Explain why a recessive allele is unable to express itself in a heterozygous state.
Answer. (a) Mendel concluded that:
(i) Characters are controlled by discrete units called factors.
(ii) Factors occur in pair.
(iii) In a dissimilar pair of factors one member of the pair dominates/only one of the parental character is expressed in a monohybrid cross in the F1 and both are expressed in the F2.
(b) The alleles are present on homologous chromosomes. The recessive allele does not code for its product or codes for a defective product. The other allele remains normal and thus expresses itself.

Question: Let us assume in a given plant the genotype symbol “Y” stands for dominant yellow seed colour and “y” for recessive green seed colour; symbol “R” for round seed shape and “r” for wrinkled seeds. Two homozygous parents (plants) with genotypes “RRYY” and “rryy” are crossed and their F1-generation progeny is then selfed. What shall be the
(a) Phenotype of F1-progeny
(b) Genotype of F1-progeny
(c) Gamete genotypes of F1-progeny
(d) Phenotypic ratio of F2 population
(e) Phenotypic ratio of yellow seed to green seed and round seed to wrinkled seed in F2 population.
Answer.
(a) Phenotype of F1-progeny: Round seeds that are yellow in colour
(b) Genotype of F1-progeny: RrYy
(c) Gamete genotypes of F1-progeny: RY, Ry, rY and ry
(d) Phenotypic ratio of F2 population: 9 : 3 : 3 : 1.
Nine round-yellow seeds; three round-green seeds; three wrinkled-yellow seeds; one wrinkled-green seed.
(e) Phenotypic ratio of yellow seed to green seed and round seed to wrinkled seed in F2 population:
Yellow seed to green seed = 3 : 1
Round seed to wrinkled seed = 3 : 1

Question: (a) Explain the phenomena of dominance, multiple allelism and co-dominance taking ABO blood group as an example.
(b) What is the phenotype of the following?
(i) I^Ai (ii) ii 
Answer. (a) Dominance: The alleles IA and IB both are dominant over allele i as IA and IB form antigens A and B, respectively, but i does not form any antigen.
Multiple allelism: It is the phenomenon of occurrence of a gene in more than two allelic forms on the same locus. In ABO blood group in humans, one gene I has three alleles IA, IB and IO/i.
Co-dominance: It is the phenomena in which both alleles express themselves when present together. We inherit any two alleles for the blood group. When the genotype is IAIB the individual has AB blood group since both IA and IB equally influence the formation of antigens A and B.
(b) (i) IAi — A blood group.
(ii) ii — O blood group.

Question: (a) State and explain the law of segregation as proposed by Mendel in a monohybrid cross.
(b) Write the Mendelian F2 phenotypic ratio in a dihybrid cross. State the law that he proposed on the basis of this ratio. How is this law different from the law of segregation?
Answer. (a) (ii) Law of segregation or law of purity of gametes
- This law states that the factors or alleles of a pair segregate from each other during gamete formation,such that a gamete receives only one of the two factors. They do not show any blending but simply remain together.
- Homozygous parent produces all gametes that are similar, heterozygous parent produces two types of gametes, each having one allele in equal proportion.
(iii) Law of independent assortment
- According to this law the two factors of each character assort or separate out independent of the factors of other characters at the time of gamete formation and get randomly rearranged in the offsprings producing both parental and new combinations of characters.
- When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.

(b) The F2 phenotypic ratio is 9:3:3:1. On the basis of this ratio Mendel proposed Law of Independent Assortment. 
(iii) Law of independent assortment.
- According to this law the two factors of each character assort or separate out independent of the factors of other characters at the time of gamete formation and get randomly rearranged in the offsprings producing both parental and new combinations of characters.
- When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.