Question

In humans, normal pigmentation is due to a dominant gene A. Albinism is due to a recessive allele a. A normal man marries an albino woman and their first child is albino. What are the genotypes of all three people? If they have more children what would their pigmentation be like?

Answer 1

Ans: A normal man marries an albino woman and their first child is albino.

Since normal pigmentation is due to a dominant allele A and albinism is due to a recessive allele a. So, for the disease aa genotype is needed. The mother is albino so the mother has genotype aa. So, mother contributed an a allele. Since the child is also albino, it's genotype is also aa. That implies that the normal father contributed another a allele. But the father has a normal pigementation. So, the father must be a heterozygous. So, gennotype of the father is Aa.

Genotype of father-Aa

Genotype of mother-aa

Genotype of child-aa

Now performing a cross between parents-

Father Aa aa mother

Now constructing a Punnett square-

Gametes	a	a
A	Aa:Normal	Aa:Normal
a	aa:Albino	aa:Albino

So, there is a 2/4=1/2 or 50% chance that their next child will be an albino. (Ans)