An explanation of the inheritance of the spider morph...

Dear VPI,

I was wondering what the story is with spider ball pythons.  I've tried a number of major breeders, and no one had the answer for me!  Since you folks are amazing at what you do, I figured I'd throw the question your way and see if you know.

Since spiders don't have a "super" form, then what would a spider x spider breeding yield?  if it were like your more standard co-doms, you would theoretically get 25% normals, 50% spiders, and 25% "super spiders".  Since obviously that 25% of "super spiders" doesn't exist, does that leave with you with 75% spiders that are all just your standard spiders?  Or perhaps that 25% of the spiders produced, though appearing normal, would have "super"-esque traits.  By this I mean that similar to the super pastel in that breeding it to a normal would produce all pastels, or in this case all spiders.  I haven't been able to find anything on this subject, and as I said I've contacted numerous breeders, and they haven't really known.

Any input you have on this question would be greatly appreciated.

Thank you, Eric

Dear Eric,

I don't know this based on personal experience--we've never bred a spider to a spider--but it's my understanding that the spider mutation is a dominant trait over a normal appearance. So when you breed a spider to a spider, you expect to see 75% of the babies with a spider appearance and 25% appear normal. Of those babies that have a spider appearance, 66% are actually het-normals and 33% are homozygous spiders. But you can't tell them apart unless you breed them.

If you breed a het-normal to a normal, then 50% are normals and 50% appear as spiders (het-normals).

But if you were to breed a homozygous spider to a homozygous normal, then you get all babies with the spider appearance ("het-normals.")

Since the spider appearance appears to be a true dominant trait, with respect to the normal trait, I'm using the term "het-normal" to refer to a snake with a spider appearance that is het for normal--in other words, the "regular" spider that is most often seen. There isn't a "super-spider" in the sense of the common use of "super" with regard to codominant traits. There ought to be a homozygous spider, though--but a homozygous dominant trait can't be told from its het form.

That is how it works if the spider is a dominant trait, at least in theory. The fact is that we have never heard of anyone creating a homozygous spider that when bred to a normal produces 100% spider babies. Surely in the 10 years that the mutation has been around, someone has accomplished it, but not that we've heard of. Maybe the homozygous form of spider is a lethal combination?

I hope that helps,  DGB