Can two parents with o type blood goups give birth to a child with a type blood group?

Question

Can two parents with o type blood goups give birth to a child with a type blood group?

Answer 1

Yes, but it's extremely rare. If at least one parent has at least one cis-AB gene, the child may test as type A. (The child is not actually type A, but may test as type A.) This is one of the reasons blood typing is no longer used to test for paternity.

Someone with one cis-AB gene and one O gene may test as O, A, AB, or B! This gene is very rare.