Quote Originally Posted by Dave86 View Post
Why should an homozygous form be letal?
Because sometimes that's just how the genetics works.

http://en.wikipedia.org/wiki/Achondroplasia

This form of human dwarfism is homozygous-lethal, but presents itself as a mere physical change for anyone heterozygous for the dwarfism gene.

There are other snake morphs out there that are non-lethal for heterozygous snakes but lethal for homozygous snakes.

These include the pearl BP (at least so far to my knowledge), and super jaguar carpet python morph (they develop as leucistic carpet pythons, and form complete bodies, but die in the egg prior to hatching).

Interestingly the jag carpet python morph causes neuro issues, similar in some regards to the spinning and wobble you get with spider ball pythons. It does not surprise me at all that any gene that causes neuro issues for heterozygous snakes might cause even more serious brain issues leading to lethality in snakes that are homozygous for the gene.