I just had an epiphany...

[Russ Lawson]

Ok, something that has never sat right with me since I started studying genetics when I first became really interested in ball python mutations around six years ago was the textbook definition and examples given for the term co-dominance. The first example was the roan cow, which basically appeared to be the same exact thing as incomplete dominance. The second textbook example of co-dominance was human blood type. This is the one that never sat right with me, and has been at least partly a mystery to me up until tonight. It was very clear to me this was not the same thing as incomplete dominance, yet every other example of something people were calling co-dominant was indistinguishable to me from incomplete dominance besides minute differences in phenotypic expression (eg: roan cow making both red and white hairs instead of pink hairs).

I had all the pieces to this puzzle for a while, but finally connected them tonight. I was long convinced there was simply no way there was only one mutation responsible for 6 different possible genotypes in blood type, so I figured at the least there were two mutations responsible for it. Then tonight, it hit me. I was thinking about allelic mutations in bed after making a post in a thread here, and the following ideas came to me all at once. Blood type O is wild-type (which I had determined a while ago), and types A and B are allelic mutations. Their mode of inheritance is dominance (AO is phenotypically identical to AA), but neither is dominant over the other, so in an individual with blood type AB, both are expressed. I'm assuming the term co-dominant refers to how types A and B are both dominant to type O, but both are expressed when paired with each other. I was actually comparing blood type to super stripe ball pythons when I thought of this. It really is the only explanation I can come up with for the genetics of human blood types that makes any sense. Anyway, I don't know if someone has come up with the same explanation before, because I've been pondering this for a very long time, and have asked multiple individuals and gotten no real explanation of what exactly the difference between co-dominance and incomplete dominance is until now. (I'm not even sure that was meant to be the difference when the term was coined - I've just assigned it to type AB blood in my mind since the term incomplete dominance described any other mutation in which the heterozygous phenotype was different from the wild-type as well as different from the homozygous mutant phenotype. By the way, I don't like the term co-dominance anymore now than I did when I first heard it. To me it still doesn't describe a different form of inheritance of a phenotype.)

If I'm wrong on this, and there's a geneticist around who knows better, I would love to be corrected - I always love a good talk about genetics - but like I've said this is the only explanation I have for human blood type. I made the comparison when I realized that AB can't reproduce itself when bred to OO (wild-type), but will only make AO and BO offspring. This is the same way that when a super stripe is bred to a normal, all you will get are spectre and yellowbelly offspring (the only difference being that yellowbelly and spectre are both examples of incomplete dominance).

Anyway, now that this is off my chest, hopefully I'll be able to shut my mind off long enough to get some sleep considering I have to be up in five hours, though I find the prospect unlikely.