Question

A woman gives birth to a son who has type O blood. She has type B blood. Which of the following can be concluded from this about her parents' blood types?

Answer 1

First of all, her genotype is not BB because she gave birth to a son with type O blood which requires two 'O' alleles (one from each parent). This indicates that her blood genotype is BO (phenotype B).

Therefore she must have inherited a B allele from one parent and an O allele from the other.

Therefore, her parents genotypes must of been either,
AB and BO <--- that's phenotypes AB and B
BO and BO <--- that's phenotypes B and B
AO and BO <--- that's phenotypes A and B
AA and BO
AO and BB

etc. etc.

The punnett squares illustrates the first two.

Also note, that the BO x BO combination has 1/2 a chance of passing on the BO genotype. Whereas the ABxBO combination has only 1/4 of a chance of passing on the BO genotype. Therefore, it would be logical to conclude that her parents genotypes were most likely BO and BO.