Question

Albinism is a recessive trait. A man and woman who both have normal pigmentation have one child out of three who has albinism (without melanin pigmentation). What are the genotypes of this child's parents

Answer 1

Both parents are heterozygous i.e. Aa

Step-by-step explanation:

This question involves a single gene coding for melanin pigmentation in humans. The dominant trait is normal pigmentation while the recessive trait is albinism. Note that, recessive trait means that both alleles must contain the same recessive alleles.

Let's say that the allele for normal pigmentation (presence of melanin) is A while the allele for albinism (without melanin) is a. The allele for normal pigmentation (A) is dominant over the allele for albinism (a). This means that allele "A" will always mask the phenotypic expression of allele "a" in a heterozygous state.

If a man and a woman with normal pigmentation gives birth to an albino child, it means the parents have the albino allele being masked in their genotype. In other words, both parents are heterozygous i.e Aa. If the two parents come together (see punnet square), gametes A and a will be produced by each parent.

The recessive alleles (a) of each parent's gamete combines to form a child that possesses the recessive trait (aa). Hence, the genotype of the parents will be HETEROZYGOUS (Aa).