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GATE 2025 BT Q.30: solution and answer

GATE 2025 · BTQ.30 · 1 mark · NAT

The allele associated with albinism in humans is recessive (c). The probability that an albino male (cc) and a carrier female (Cc) will have an offspring with normal skin pigmentation is ________ . (Round off to one decimal place)

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Answer and solution

ANSWER

The probability of normal-pigmented offspring is 0.5.

Official answer: 0.5Source: Official answer key, GATE 2025 BT (IIT Roorkee)

  1. Set up the cross: albino male (cc) × carrier female (Cc).

  2. Determine gametes: the male, being homozygous recessive, produces only c gametes. The female, being heterozygous, produces C gametes with probability 1/2 and c gametes with probability 1/2.

  3. Combine gametes to get offspring genotypes: c (from father) with C (from mother, prob 1/2) gives Cc; c (from father) with c (from mother, prob 1/2) gives cc. This is a standard test-cross, yielding a 1:1 genotypic ratio.

  4. Assign phenotypes: since c is recessive, Cc offspring show normal skin pigmentation (C is dominant and masks c), while cc offspring are albino.

  5. Probability of an offspring with normal pigmentation = P(Cc) = 1/2 = 0.5.

KEY IDEA

A test cross between a homozygous recessive individual (cc) and a heterozygous carrier (Cc) produces offspring in a 1:1 ratio of heterozygous (Cc, normal phenotype) to homozygous recessive (cc, albino) genotypes, since the recessive parent contributes only the c allele and the heterozygous parent contributes C or c with equal probability.

WATCH OUT FOR

Confusing this test cross with a standard Cc × Cc monohybrid cross (which gives a 3:1 phenotypic ratio) and answering 0.75 instead of recognizing that one parent is homozygous recessive, giving a 1:1 ratio and an answer of 0.5.

From the GATE 2025 BT paper. Every question of this paper · Download the full paper · All 10 Mendelian genetics questions, with notes

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