Heredity MCQs

Q1. Consider the following statements about sex determination in humans: (I) Human females are XX. (II) The father determines the sex of the child. (III) Every egg carries a Y chromosome. Which of the above statement(s) is/are correct?

• A.None of these
• B.Only III
• C.Only I
• D.Only I and II

Answer: D. Only I and II

NCERT fact (biology, chapter 'Heredity'): An ionic bond forms when a metal donates electrons to a non-metal: e.g. Na -> Na+ + e-, Cl + e- -> Cl-, and Na+ + Cl- -> NaCl; the resulting solid is held by strong electrostatic attraction => this fixes the property asked in the stem; hence the answer = Only I and II. Why option D fits - Correct. Distractor analysis: option A) 'None of these' is incorrect: Re-check each statement individually; option B) 'Only III' is incorrect: Re-check each statement individually. Final answer - D) Only I and II.

Q2. In a dihybrid cross, two pea plants heterozygous for both seed shape and seed colour (RrYy) are crossed. What is the ratio of phenotypes in the F2 generation?

• A.9 : 3 : 3 : 1
• B.3 : 1
• C.1 : 1
• D.9 : 7

Answer: A. 9 : 3 : 3 : 1

Spot-the-setup - a typical biology numerical. The data on the table: 2. We are after What is the ratio of phenotypes in the F2 generation?. Pick the chapter relation that contains every given quantity and the unknown; rearrange for the unknown. Common-sense check: When two plants heterozygous for two traits (RrYy × RrYy) are crossed, the two gene pairs assort independently. Lock in option A) 9 : 3 : 3 : 1. Trap-watch: option B) '3 : 1' doesn't hold - 3:1 is the ratio for a monohybrid (single-trait) cross, not a dihybrid one; option C) '1 : 1' misses the point - 1:1 is a test-cross ratio, not the F2 dihybrid ratio.

Q3. Consider the following statements about Mendel's work: (I) Monohybrid F2 phenotypic ratio is 3:1. (II) Dihybrid F2 phenotypic ratio is 9:3:3:1. (III) F1 plants show the recessive trait. Which of the above statement(s) is/are correct?

• A.None of these
• B.Only III
• C.Only I
• D.Only I and II

Answer: D. Only I and II

Given 2, 3, 1, 2, 9, asked for the unknown. By hence the answer = Only I and II. Mendel's monohybrid cross TT x tt -> F1 all Tt (tall). Thus option D) Only I and II. Others fail: option A) 'None of these' doesn't hold - Re-check each statement individually; option B) 'Only III' misses the point - Re-check each statement individually.