A woman who has blood type A positive has a daughter who is type O positive and a son who is type B negative. Rh positive is a trait that shows simple dominance over Rh negative and is designated by the alleles R and r, respectively. A third gene for the MN blood group has codominant alleles M and N.

Which of the following is a possible phenotype for the father?

A) A negative

B) O negative

C) B positive

D) AB negative

E) impossible to determine

The answer is C) B positive.

A mother has either AA or AO genotype. Since her daughter has OO genotype, the one O allele is from a father and one is from the mother. Thus, mother must have allele O. So mother's genotype is AO.

A mother with AO genotype will pass to her son either A or O allele. Since son is type B negative, it could inherit from mother only O allele. Son's genotype is BO. This means that he inherited from the father B allele.

Father has either BO or BB genotype. Since his daughter has OO genotype, she must inherit one O allele from the father. Thus, the father has BO genotype.

Talking of Rh, her son is type B negative. A negative Rh phenotype contains both recessive alleles r, so the mother must have allele r (she is either Rh + heterozygous, or Rh - homozygous). The same is with father - he is either Rh + heterozygous, or Rh - homozygous.