Quote Originally Posted by RandyRemington View Post
If one wild type copy of a homozygous lethal mutation can keep the animal alive wouldn't that count as a blend? At least with HG Woma we get to see that the pearl phenotype is different before it dies (with the one possible recent exception). Maybe a homozygous spider would look different if we got to see it?

I honestly don't know where the line is drawn in terms of semantics (incomplete dom versus dominant) for lethal genes. I think a lot of it is just convention, because we're finding it more and more difficult to fit the complex "round pegs" of molecular genetics into the "square holes" of the simple Mendelian classifications systems.

In my mind, I could justify the semantic classification that a homozygous lethal gene that kills the neonate after birth (like the pearl) could be considered to be incomplete dominant, whereas one that was incompatible with life and did not allow the embryo to form very far past fertilization could be potentially considered simple dominant (since there is no homozygous "phenotype" to compare it to, it's just a nonexistent entity). Really though, I think that's kinda stretching it and I'm just trying to justify the nomenclature as I've seen it used. (I have also seen suggestions that we may classify different pleiotropic effects of the same gene differently; for example, we could classify the pattern part of the woma gene as incomplete dominant and the lethal neurologic part as simple recessive, even though they're assumed to be the same gene. Confusing!)

BTW, in looking this up and doing more research into the lethal white foal syndrome, evidently that was a bad example for a simple homozygous lethal trait -- apparently, there is huge debate as to how it is linked to the overo coat pattern, and whether it's even a pleiotropic effect of the overo gene at all. There's also a lot of politics involved, apparently ... Guess we shouldn't feel so bad about all the confusion surrounding the mutations we work with -- I'd say that we're not doing so bad trying to figure it out on our own (ie, without any research or funding for it!)