A man heterozygous for albino gene marries a woman who is also heterozygous for the gene. Both have normal…

Analyze the question

Albinism is a recessive trait. Both parents are heterozygous carriers (Aa), so they have normal skin colour but each carries one albino allele (a). We need the probability that a child is albino (aa).

Concept

This is a monohybrid cross between two heterozygotes. Each parent produces two types of gametes, A and a, in equal numbers. An albino child appears only when the child receives the recessive allele (a) from both parents.

Solution

Parents: Aa × Aa

Punnett square:

| | A | a | |---|---|---| | A | AA | Aa | | a | Aa | aa |

Genotype ratio: 1 AA : 2 Aa : 1 aa Phenotype ratio: 3 normal : 1 albino

Probability of an albino child (aa): P(a from father) × P(a from mother) = 1/2 × 1/2 = 1/4

Why the other options are wrong

• A (1/3): this wrongly counts only the 3 normal-looking genotypes, not all 4 outcomes.
• C (4/4): this would mean every child is albino, which is certain and not possible here.
• D (2/4): this is the probability of a carrier child (Aa), not an albino child.

Exam tip

When two carriers (heterozygotes) cross, remember the 3:1 phenotypic ratio. The recessive trait always shows up in 1 out of 4 children, which is a 25% chance. Each pregnancy is an independent event, so having one albino child does not change the chance for the next.