About This Section
This section collects CBSE and other board style questions on Heredity, grouped by marks. Learn the pattern, the mark distribution and the expected answer style.
Study Strategy:
- Solve each cross with a Punnett square first.
- Match answer length to marks.
- Show ratios and genotypes clearly.
- Practise all NCERT exercises.
1-Mark Questions (MCQ / Very Short)
Q1. [CBSE] The F2 ratio in a monohybrid cross is — (a) 1:1 (b) 3:1 (c) 9:3:3:1 (d) 1:2:1 (phenotype) Solution: (b) 3:1.
Q2. [CBSE] In humans, the sex chromosomes of a female are — (a) XY (b) XX (c) YY (d) XO Solution: (b) XX.
Q3. [CBSE] A section of DNA that codes for one protein is a — (a) chromosome (b) gene (c) allele (d) nucleus Solution: (b) gene.
Q4. [CBSE] The genotype of blood group O is — (a) I^A i (b) I^B i (c) I^A I^B (d) i i Solution: (d) i i.
1-Mark Questions (continued)
Q5. [CBSE] Name the scientist regarded as the father of genetics. Solution: Gregor Johann Mendel.
Q6. [CBSE] How many pairs of autosomes are present in humans? Solution: 22 pairs.
Q7. [CBSE] A tall plant with genotype Tt is crossed with tt. Give the phenotype ratio. Solution: 1 tall : 1 short (1:1).
Q8. [CBSE] Which parent determines the sex of a human child? Solution: The father (through the X or Y sperm).
2-Mark Questions
Q9. [CBSE] Distinguish between dominant and recessive traits with an example. Solution: A dominant trait is expressed even with a single copy (e.g. tall, T); a recessive trait is expressed only when both copies are present (e.g. short, tt).
Q10. [CBSE] Differentiate between genotype and phenotype. Solution: Genotype = the genetic make-up (letters), e.g. Tt. Phenotype = the visible characteristic (appearance), e.g. tall.
2-Mark Questions (continued)
Q11. [CBSE] Why did Mendel choose the garden pea for his experiments? Solution: It has clear contrasting characters and can be self- or cross-pollinated, allowing controlled breeding.
Q12. [CBSE] How is variation useful for the survival of a species? Solution: When the environment changes, some variants are suited to the new conditions and survive and reproduce, keeping the species alive.
Q13. [CBSE] Why must a gamete carry only one set of genes? Solution: So that the number of gene sets stays constant — fertilisation of two single-set gametes restores the normal two sets.
3-Mark Questions
Q14. [CBSE] How do Mendel's experiments show that traits may be dominant or recessive? Solution: In F1 (Tt) only tallness appears — tallness is dominant, shortness is hidden. In F2, shortness reappears in 1/4 of the plants (3:1), showing it was recessive, expressed only as tt. Hence traits can be dominant or recessive.
Q15. [CBSE] With a cross, explain the 3:1 F2 ratio of a monohybrid cross. Solution: Tt × Tt → gametes T, t from each. Punnett square gives TT, Tt, Tt, tt. TT and Tt are tall, tt is short → 3 tall : 1 short; genotype 1:2:1.
3-Mark Questions (continued)
Q16. [CBSE] How do Mendel's experiments show that traits are inherited independently? Solution: In a dihybrid cross (round-yellow × wrinkled-green), the F2 shows new combinations — round-green and wrinkled-yellow — not present in the parents. This proves seed shape and colour are inherited independently (9:3:3:1).
Q17. [CBSE] Explain how the sex of a child is determined in human beings. Solution: Females are XX, males XY. The mother gives an X to every child. The father gives X (→ girl, XX) or Y (→ boy, XY). So the father's chromosome decides the sex; the ratio is about 1:1.
Q18. [CBSE] A man of blood group A marries a woman of group O; their child is group O. Can we decide which allele is dominant? Explain. Solution: No. The A father may be I^A i; I^A i × ii gives A or O children whether or not A is dominant. One family cannot establish dominance.
5-Mark Questions
Q19. [CBSE] (a) What are dominant and recessive traits? (b) Using T (tall) and t (short), show a monohybrid cross up to the F2 with genotype and phenotype ratios. Solution: (a) Dominant = expressed with one copy; recessive = expressed only with both copies. (b) P: TT × tt → F1: all Tt (tall). F1 × F1 (Tt × Tt) → F2: TT, Tt, Tt, tt. Genotype 1:2:1; phenotype 3 tall : 1 short.
Q20. [CBSE] Explain, with a cross diagram, sex determination in humans and state who determines the sex of the child. Solution: Mother XX → all eggs carry X. Father XY → half sperms X, half Y. X-sperm + X-egg = XX (girl); Y-sperm + X-egg = XY (boy). Since the mother always gives X, the father's chromosome (X or Y) determines the sex; the ratio is about 1:1.
5-Mark Questions (continued)
Q21. [CBSE] Explain how genes control a trait, and why each organism carries two copies of each gene on chromosomes. Solution: A gene makes a protein/enzyme; for height, the enzyme controls the amount of growth hormone (efficient enzyme → tall; altered gene → short). Each organism inherits one gene set from each parent, so body cells carry two copies of each gene, present on paired chromosomes. Germ cells carry one of each pair; fertilisation restores the pair, keeping the chromosome number stable and ensuring equal contribution from both parents.
Q22. [CBSE] (a) State Mendel's dihybrid result. (b) What does the 9:3:3:1 ratio prove? Solution: (a) RrYy × RrYy → F2 phenotypes 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green. (b) The new combinations (round-green, wrinkled-yellow) prove that seed shape and colour are inherited independently — Mendel's law of independent assortment.