Question

Rh factor is a gene with 2 alleles, Rh positive, which is dominant to Rh negative. The gene that codes for Rh factor independently assorts from the ABO gene. A female who has A blood and is Rh positive (A positive) and whose mother is O negative marries a man that is O negative. What is the probability that they will have a child that is O positive?

Answer 1

Answer:

1/4

Explanation:

• The Rh+ allele is dominant to Rh-.
• The O allele is recessive to the A allele.

The woman has an A blood type (she could be either AA or AO), but we know her mother had a type O blood, so she had two copies of the O allele. Therefore, the woman inherited one and her genotype for the ABO gene is AO.

The same thing can be said about the Rh gene. The woman is Rh+, so she could be either homozygous for the Rh+ allele or heterozygous. But because her mother was Rh negative, which is recessive, we know that she must have inherited a Rh- allele from her mother and is therefore Rh+ Rh-.

The man is O negative, so his genotype is completely recessive for both genes (OO Rh- Rh-).

If you do a Punnett Square to find out the possible genotypes of their offspring (please see attachment), you'll get that the probability of having an O positive child (OO Rh+_) is 1/4.