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Inherited Disorders and Birth Defects

How is My Blood Type Possible?



My father sometimes tests as type A, and sometimes type AB. This would suggest he has B antigens, but they`re weak and don`t always show up. My mother is type B.

I`m type O positive. Assuming my father is my father, how is this possible?


If your father actually has AB blood type (gene = AB) and your mother is type A (gene = AA or AO) – the only possibilities should be blood types: A, B, or AB. Thus if your father does indeed have AB blood type, then he would not be your biological father if you are type O.

Another explanation, although it is very rare, is the possibility that there was a change (mutation) in the A form of the gene (allele) and switched it to an O, which you inherited. This can explain some of the rare outcomes that seem to defy the common genetic rules.

There is a really good explanation of how this type of mutation can occur at the website below that was written by the geneticists at Stanford University.

Another possible explanation is that you have been mistyped. The main blood groups of the ABO blood group system (A, B, O and AB) all have various subgroups. More of these are being discovered as techniques, equipment and genetic testing have become available. The A blood type has the most variation in subgroup of any of the ABO blood types with about 20 different known subgroups. The failure to detect a weak subgroup of A may result in a patient or donor being mistyped as group AB or B or O.

Related Resources:

The Tech Museum Stanford University

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Response by:

Anne   Matthews, RN, PhD Anne Matthews, RN, PhD
Associate Professor of Genetics
School of Medicine
Case Western Reserve University