Question

If a mom who has type A blood & a dad who has type O blood have a child with type B blood, what has happened?

Answer 1

If the child is the birth child of the mother (not adopted), it means that someone other than the man with type O blood is the biological father.

Explanation:

Since genes exist in pairs, there are two possibilities for the mother - AA or AO - both of which will cause her to be typed as A group.

The father, being O group, will have only OO genotype as only that results in the absence of A or B surface antigen.

The only way a child from the stated mother could have B group is if the mother is genetically AO and the father has at least one gene for B (BB or BO) so that the child could receive O from the mother and B from the father in order to be typed as B group.