Then take a genetics class.
Things are very simplified on the web. I don't know why I try to tell you these things that is beyond your comprehension.
But the original post says the gene is "simple dominant" which is incorrect. There is no simple dominant. There are multiple types of dominance and it's never just that simple.
I still stand on that the spider gene is recessive lethality. But here's some 'web' stuff you can read. Since everyone seems to be linking wikipedia:
http://en.wikipedia.org/wiki/Lethal_alleles
http://www.microbiologyprocedure.com...al-alleles.htmIf the mutation is caused by a recessive lethal allele, the homozygote for the allele will have the lethal phenotype.
There is more on that website about some other types of inheritance.
Until someone proves otherwise by proving there is a homozygous spider, this is likely the case.