Ask Dr. H: Why son's blood type can differ from parents'

Question:

Recently, our son needed an operation and had his blood type identified as Type O. The problem I'm having is that my wife and other son, as well as I, are Type B. How is it possible that both parents having the same blood type can result in a child having a completely different blood type?

Answer:

That's easy. When we refer to blood types like A, B and O, we're describing the presence or absence of A or B antigens - protein substances found on the surface of your red blood cells. A person who is Type O has no A and B antigens on his red blood cells.

Rh is another antigen that may or may not be on the surface of red blood cells. If we have that antigen, we're called Rh positive; if we don't, we're Rh negative. A, B, and Rh are the major antigens that may be present on red blood cells, but there are nearly 600 other minor antigens to which some patients might react during a blood transfusion.

When people get their blood typed, it's either A+ (the positive means Rh is present); A-; B+; B-; O+; O-; AB+; or AB-. Every person inherits half the genetic information that determines blood type from his or her mother and half from the father. If someone is Type A, he or she may have inherited an A antigen from each parent to be genetically AA, or may have inherited a single A antigen from one parent and no antigen from the other parent to result in a genotype of AO.

Both AA and AO are considered type A. The same scenario applies for Type B.

If a person is Type O, two possibilities exist: Either both parents are Type O and genetically OO (no A or B antigens on the red cell surface) or neither parent is pure genetic A or B, but rather genetically AO or BO - with each one passing on an O so that the child ended up as Type O (genetically OO).

In your situation, both you and your wife would be Type B and genetically BO in order for each of you to pass on genetic information for one of your O's to your son, making him genetically OO - Type O.