Answer by Diane Eager
This combination is possible if the father has a rare condition named Bombay phenotype. This is the lack of a molecule, called H, which connects the A and B molecules to a blood cell’s surface. As a result the blood cells without H do not have A or B molecules on their surface even if the person has the gene for the A or B. Therefore, their blood cells react to a blood group test as if the person’s blood was Group O. However, people with this condition can still pass on the A or B genes to their children. If the children inherit a normal gene for H from their other parent, these children can be A or B blood group, or AB.
In case anyone is wondering, blood groups are no longer used to determine parentage now that we have DNA profiling. Also, Bombay phenotype is a rare condition, but there is a generally available blood test for it.
For more information on blood groups, see the question:
How did A B AB O & Rh blood groups come from Adam & Eve? What blood group were they? Answer here.
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