Tosha,
First I just wanted to say that I've read alot of awesome posts from you on these boards and I don't want you to take this as an attack, BUT...
1) First your evidence for this trait not being genetic is scant at best, "defect does not seem to be genetic in that a snake with the defect does not necessarily mean it will pass on its offspring and 'healthy' spiders also seem to pass on the defect," all that what your saying above means is that the spider trait is most likely not linked directly to the spider gene or a result of it, although we can't rule that out for certain without spending five years on a breeding project proving or disproving it.
Personally I think that most likely in my opinion is that this is a gene seperate from the spider gene but that originated in one of the early breedings or even with the original spider but that has been distributed rapidly since the dominant nature of the spider gene makes it easy produce large amounts of animals without inbreeding. This in and of itself is probably why this problem hasn't been so widely known as there have probably only been one or two handfuls of spiderXspider breedings since either the homozygous condition is either lethal or undifferentiable from the heterozyous phenotype.
As to the nature of this phenomenon if it is genetic and inheritable, it is hard to say although I would probably most likely have to say recessive although there is scant evidence either way, codominant also being just as likely. Like I have said before, the only way to know is for someone to take some of these affected animals and breed to prove or disprove this phenomenon.
2) Comparing selective breeding (what all those wonderful breeders out there with rooms full of ball pythons are doing) with human beings with blindness having children is not ok in my book. I work in a hospital and we deal with congenital defects in babies all the time. Last year I remember a family with 8 children , both parents were heterozygous for congenital heart defects, and out of the children all 8 were girls, and 3 of them had said defects. The wife was pregnant with the 9th child, and I wondered why the parents would keep having children that they knew had a 1/4 chance of being born with defects. Well I came to find out later that the reason they kept having children was because the parents wanted a boy. Is it really ok for parents knowingly to bring children into this world that are going to endure years of painful surgeries, and that if they didn't receive medical care immediately upon birth they would die? I think not, but in our world it is infinitely harder to control who can have a gun, buy alcohol, or drive a car than it is to control who can have a child.
Well, anyhow Tosha just wanted to reply and give the devil's advocate opinion as to what you said, in no way is this a personal attack, I just don't like people dismissing something that is potentially very very serious (hint: notice no one from any of the big breeders has replied to this post, nor any of the medium breeders) and I thought I'd give you the flip side since I've read many of your posts and have gained insight from them ball python husbandry, as you always have words for everyone who comes on here, and more likely than not help the person and their reptile at the same time.
Well anyhow, sorry this was so long, have a good one and feel free to refute me if ya like,
-Dan
Poor Ankin he'll just have to be celebate for a while longer. 