Question Bank — Principles of Inheritance and Variation

Section A — MCQs (10 questions, 1 mark each)

1. Which of the following contrasting traits was not studied by Mendel in pea plants?
(a) Stem height – Tall/Dwarf
(b) Flower colour – Violet/White
(c) Seed shape – Round/Wrinkled
(d) Leaf shape – Broad/Narrow

2. In Mendel’s monohybrid cross between tall (TT) and dwarf (tt) pea plants, the phenotypic ratio obtained in F₂ generation is:
(a) 1:2:1
(b) 3:1
(c) 9:3:3:1
(d) 1:1

3. The Law of Segregation is based on the fact that:
(a) Alleles blend in heterozygotes
(b) Both alleles are recovered unchanged in F₂ generation
(c) Only dominant allele is transmitted to gametes
(d) Factors always remain homozygous

4. In the plant Snapdragon, a cross between red-flowered (RR) and white-flowered (rr) plants produces pink-flowered F₁ progeny. This is an example of:
(a) Complete dominance
(b) Incomplete dominance
(c) Co-dominance
(d) Multiple allelism

5. ABO blood groups in humans are controlled by:
(a) Two alleles showing complete dominance
(b) Three alleles showing co-dominance and dominance
(c) Polygenic inheritance
(d) Sex-linked genes

6. Morgan’s experiments on Drosophila showed that genes for yellow body and white eyes are:
(a) Loosely linked
(b) Tightly linked with very low recombination
(c) Located on autosomes
(d) Showing independent assortment

7. In humans, sex determination is of:
(a) XO type with male heterogamety
(b) XY type with male heterogamety
(c) ZW type with female heterogamety
(d) Haplodiploid type

8. Sickle-cell anaemia is caused by:
(a) Frame-shift mutation
(b) Point mutation substituting glutamic acid by valine at 6th position of β-globin chain
(c) Deletion of an entire chromosome
(d) Polyploidy

9. Assertion (A): A test cross is used to determine the genotype of an organism showing dominant phenotype.
Reason (R): In a test cross, the organism is crossed with the recessive parent and progeny ratios reveal whether the dominant parent is homozygous or heterozygous.

10. Assertion (A): In polygenic inheritance of human skin colour, the phenotype is the additive effect of alleles.
Reason (R): Skin colour is controlled by three or more genes and is also influenced by environment.

Section B — Very Short Answer (6 questions, 2 marks each)

1. Define allele and differentiate between homozygous and heterozygous conditions with an example from Mendel’s pea experiments.

2. What is a test cross? Why did Mendel perform it?

3. Differentiate between dominance and incomplete dominance with one example each from the chapter.

4. State the chromosomal theory of inheritance as proposed by Sutton and Boveri.

5. What are multiple alleles? Give one example from the chapter.

6. Differentiate between α-thalassemia and β-thalassemia on the basis of the chromosome involved.

Section C — Short Answer (5 questions, 3 marks each)

1. Explain Mendel’s Law of Dominance with the help of a monohybrid cross between tall and dwarf pea plants.

2. Describe incomplete dominance using the inheritance of flower colour in Snapdragon. How does the phenotypic ratio differ from a typical Mendelian monohybrid cross?

3. Explain co-dominance with reference to ABO blood groups in humans. How many genotypes and phenotypes are possible?

4. What is linkage? How did Morgan’s experiments on Drosophila demonstrate linkage and recombination?

5. Describe the sex-determination mechanism in honey bees. How is it different from the XY mechanism in humans?

Section D — Long Answer (3 questions, 5 marks each)

1. Describe the dihybrid cross performed by Mendel between pea plants differing in seed colour and seed shape. Explain the Law of Independent Assortment. Show the results using a Punnett square and state both phenotypic and genotypic ratios obtained in F₂ generation. (Include labelled Punnett square description.)

2. Explain the chromosomal mechanism of sex determination in humans. Describe the inheritance pattern of sex-linked traits with one example. Why is it incorrect to blame mothers for the birth of a female child?

3. What is pedigree analysis? Draw a representative pedigree chart (labelled) showing the inheritance of an autosomal recessive trait (e.g., sickle-cell anaemia) and an X-linked recessive trait (e.g., colour blindness). Explain how such analysis helps in tracing Mendelian disorders.

Section E — Case/Source-Based (2 questions, 4 marks each)

Case 1:

In humans, ABO blood grouping is controlled by the gene I with three alleles Iᴬ, Iᴮ and i. Iᴬ and Iᴮ produce slightly different sugars on RBC surface while i produces none. Iᴬ and Iᴮ are co-dominant while both are dominant over i.

Sub-questions:
(a) How many different genotypes and phenotypes are possible?
(b) A person with blood group AB marries a person with blood group O. What are the possible blood groups of their children?
(c) Why is this an example of both multiple allelism and co-dominance?
(d) If a heterozygous Iᴬi person donates blood, which blood groups can receive it safely?

Case 2:

Sickle-cell anaemia is an autosomal recessive disorder caused by substitution of glutamic acid by valine at the 6th position of the β-globin chain. Homozygous HbˢHbˢ individuals show the disease while heterozygotes show sickle-cell trait.

Sub-questions:
(a) What is the probability of a carrier couple having a diseased child?
(b) How does the shape of RBC change under low oxygen tension and why?
(c) Differentiate between sickle-cell anaemia and thalassemia.
(d) Why is this considered a qualitative defect while thalassemia is a quantitative defect?