Question

A couple believed that they have brought the wrong baby home from hospital. The wife is of blood group $O$ , her husband of group $B$ and child of group $O$ . Could baby be theirs?

Answer 1

Human blood grouping is controlled by three alleles I${}^A$, I${}^B$ and i. I${}^A$ and I${}^B$ are codominant with each other but dominant over i.

Since the wife has blood group O, this means her genotype is ii.

On the other hand, the husband has blood group B, this means his genotype is either I${}^B$i or I${}^B$I${}^B$.

As the blood group of the baby is O, this means his genotype is ii.

One of the two i alleles is inherited from mother and the other one from father. Hence, there are chances that the baby could be theirs.