The Science of Desperate Housewives

Okay, this has been bugging me for almost a week. I just have to clear this up.

On Sunday's episode of Desperate Housewives, we found our favorite ladies at the Wisteria Lane Blood Drive. Zack walked up to the registration table and told the nurse that his blood type was AB-. The woman remarked, correctly, that this was a rare blood type. She then said that another man with that same blood type had just donated. The man of whom she spoke was -- you guessed it -- Mike.

What was the purpose of this sanguine exchange? It was supposed to be a clue to Zack that Mike is his biological father. Sadly, the writers really screwed the pooch. Here's why.

There are two different antigens used to type blood for donation and transfusion, the ABO antigen and the Rhesus (Rh) antigen, antigens being proteins recognized by antibodies. The ABO blood antigens are coded for by three different alleles, the Rh antigen by two alleles. Alleles are simply different genetic sequences that encode the same protein. In basic biology, alleles are often referred to as "dominant" or "recessive." For the Rh antigen, Rh+ blood type (expressing the Rh antigen) is "dominant" while Rh- blood type (failure to express the Rh antigen) is "recessive." Therefore, if an individual receives one Rh+ allele from his father and one Rh- allele from his mother, then that individual will be blood type Rh+, even though he only has one Rh+ allele. To be Rh-, one must receive an Rh- allele from both parents.

ABO blood typing is a little different. At any given genetic locus, we all have two alleles, one inherited from our father and the second from our mother. This means several combinations are possible -- two A alleles, two B alleles, two O alleles, or a combination (AB, AO, BO). Because the A and B alleles are co-dominant, if one has both an A and a B allele, then one's blood type is AB (one expresses both the A and B antigens). If an individual's blood type is O, then that person has two O alleles, and must have neither an A nor a B allele (and therefore expresses neither the A nor the B antigen).

So, if I were a writer on DH, and I wanted to use blood typing to show that a character's custodial father was not his biological father, this is what I would do. I'd ignore the Rh antigen completely because it's a simple recessive/dominant situation, so it's not very informative. I would simply make the child's blood type O and I would make his custodial father's blood type AB. An AB father cannot parent a child with blood type O, barring spontaneous mutations. This is because he would pass either the A or B allele to all of his offspring, and since the A and B alleles are always dominant over the O allele, any resulting children would always be either A, B or AB.

Unfortunately, this would have required explanations such as the one I've presented here. Maybe that's why I work in a research lab and don't write for television shows.
But at least I've gotten this off of my chest! Oh, and if anyone really wants to "see the math", then I'll be happy to send you a Punnett square. Or perhaps you could go hide under the bed until the feeling passes.