Question

Blood type in humans is an example of trait that has multiple alleles. The alleles for type A blood and type B blood are codominant, and type O blood is recessive to both type A and type B. If a girl with type AB blood has a brother with type O blood and a sister with type B blood, which of the following genotypes could it be possible for their parents to have?

AO and BO
AB and OO
AB and BO
AO and AB

Answer 1

The first statement.

Answer 2

Why is that?

Answer 3

Since O is ressesive you need two O alleles in order to produce the phenotype of type O blood. As there are also children with AB and B blood the parents must carry those alleles as well. Parents with type AO and BO is the only option here that would give these results.