My Mother's Blood Type Is O+ And My Father’s Blood Type Was B+ And Mine Is AB+. Is That Possible?

Many books says that it's not possible, can you tell me if this is true?

This question was originally posted on Quora.com and was answered by Alexander Streng, PhD, Clinical Chemistry Resident.

The ABO blood group consists of a single gene with 3 alleles (very unoriginally called A, B, and O). The A and B alleles are dominant while the O allele is silent. This means that when the blood group is O (like your mother’s), the genotype has to be OO. When the blood group is B (like your father’s), the genotype can be either BB or BO (Blood group A is similar with either the AA or AO genotype). Lastly, when the blood group is AB, the genotype has to be AB.

When you know this, we can instantly see the problem. Assuming these results are correct, the genotype of your mother is OO and she will always pass the O allele to her offspring. The genotype of your father can be either BB or BO. We don’t know which, but it doesn’t matter in this situation. He will either pass on the B or the O allele.

So, as your parent’s child, your genotype will have to be either BO or OO (Blood group O or B). A and AB are not an option. There are several other complicating factors, but basically, it is as simple as this.

In practice, the reason is almost always that one of these three blood groups are not correct. Have them analyzed again in the same laboratory if you want to know for sure.

There are some rare occurrences that might explain this. One possibility that people always mention is that your mother is of the “Bombay” phenotype. This is a rare genetic occurrence in which a precursor substance of the blood group antigens is absent. The a priori chance of this happening is 1 in a million for caucasians and 1:10,000 for indians. It is completely benign, but it would result in the O blood group phenotype while potentially having a different genotype.

The other would be a chimaera, which is a very rare occurence in which an individual has 2 completely separate sets of DNA. This could in theory explain these results but is even more unlikely than the Bombay variant above.

Barring a newborn switch-up at the hospital (which is a real possibility in some countries, btw) or an adoption, you can rule out that your father is not your biological father based on these results. The blood groups that do not match with one another are yours and your mother’s. Since your mother has neither the A or B alleles and you do not have the O allele there is a clear discrepancy. The most likely explanation is that either of you do not have the blood group you think you have. Have your blood groups retested in the same laboratory if you realy want to know.

If you truly have doubts whether or not your parents are your biological parents, you should ask for a DNA test, not a blood group. Or it might be easier (and cheaper) to just talk with them about it.