Expert: Dana Krempels, Ph.D. - 8/16/2008

Question
My friend (female) has AB blood type and her ex (male) has O+ blood type. Could it be possible that there daughter could have B- blood type and some kind of RH factor. I always heard that your kids would be one parent or the other. And how does someone get this RH- Factor.

Answer
Dear Melinda.

The ABo blood groups are so named for a specific protein (an immunoglobin protein) embedded in the plasma membrane of the red blood cells.  A person with type A blood has type A immunoglobin, a person with type B has type B immunoglobin, a person with type AB has both, and a person with type O has neither A nor B protein.  (The physical difference between these immunoglobins is very slight--a change of the sugar at one terminal end of the glycoprotein.)

In terms of genetics, the genes coding for A and B immunoglobins are dominant to those encoding the O condition.  A and B are codominant, meaning that if both are inherited, both will be expressed.  (A person inherits two copies of every gene:  one from mom, and one from dad.)

If your female friend has type AB blood, then her ova will have genes that encode *either* the A immunoglobin OR the B immunoglobin--not both.  She is heterozygous for the ABo proteins, meaning she has one copy coding for the A protein and one copy coding for B.

The male in the picture produces neither protein.  A child produced by and AB x O cross will have either type A blood or type B blood.  Those are the only two possibilities.

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The Rh factor is yet another protein inserted into the plasma membrane.  A person with Rh+ has the protein, whereas a person with Rh- does not.  The gene coding for Rh+ is dominant to the gene coding for Rh-.  A person with Rh- blood (whatever the ABo group is; they can combine randomly) can produce gametes ONLY with Rh- genes.

A person who is Rh+ can have one of two genotypes:  BOTH the genes are Rh+, or there may be one Rh+ and one Rh- gene.  If the latter case is true, then a child can inherit either the Rh+ or the Rh- gene.

If the O+ father had an Rh genotype of Rh+/Rh-, he could still be the father of the child.  You don't say what your friend's Rh factor is, but even if she's positive, she, too, could be a carrier of the Rh- gene, and pass it on to her child.

So the answer is that yes, this blood type could come from parents with these blood types.

Hope that's not too confusing!

Dana