At one gene locus in humans there are 2 alleles: one for curly hair and another for straight hair. Neither allele is dominant, and heterozygous individuals have wavy hair. At a second gene locus, 3 alleles control ABO blood type. The alleles at these 2 gene loci assort independently. A man with straight hair and A blood has children with a woman with wavy hair and B blood. Their first child has straight hair and O blood. If their second child is a boy with straight hair, what is the probability that he has A blood? (Express your answer as a percentage. Enter the number only without the percent sign. For example, enter 100% as 100 and enter 12.5% as 12.5)

25%

Explanation:

Genotypes and phenotypes:

SS = straight hair CC = curly hair SC = wavy hair AA or A0 = type A blood BB or B0 = type B blood AB = type AB blood 00 = type 0 blood

A man with straight hair and A blood has the genotype SS AA or SS A0.

A woman with wavy hair and B blood has the genotype SC BB or SC B0.

Their first child has straight hair and O blood: SS 00

Because the child has type 0 blood, both parents must have a 0 allele, so their genotypes are SS A0 and SC B0.

If their second child is a boy with straight hair, what is the probability that he has A blood?

The hair and blood type genes are independent, so the hair phenotype of the child is irrelevant to determine the probability of him having A blood.

The parental cross for blood type is A0 x B0.

If you do a Punnett Square, you'll get the following offspring:

25% AB, 25% B0, 25% A0 and 25% 00.

The answer is the child has a probability of 25% of having A blood.