Does blood type determine paternity?
It doesn’t prove much. The only thing which is weird for an O child is if their father is AB. Every person has their ABO blood type determined by 2 alleles (gene versions), one from each parent. O types have to have two O alleles; AB have 1 A and 1 B (A types may have a hidden O, as long as their other allele is an A, same for B) and so can’t pass on an O to their children. Assuming this is your child, he’s received 1 O allele from you, and 1 from mum. The rhesus grouping is also due to 2 alleles, but is simpler. Basically a positive child needs at least 1 positive parent. So the blood info is certainly in favour of your being the father, but nothing like proof.
