The genetics wizard is apparently assuming spider is a dominant mutation because that's what the founder says it is although I think he admits he didn't do the breedings to actually prove that. Many of us speculate that the lack of a public proven homozygous spider could be caused by the mutation actually being homozygous lethal which would technically be co-dominant. However, if that is the case it would be pretty much impossible to ever prove; we would just go on and on with no answer like we have now for 20+ years.

Pinstripe on the other hand has actually been proven to be a dominant mutation. The pinstripe founder reports producing a homozygous pinstripe that looks like the regular heterozygous pinstripes but proved homozygous by producing a large number of only pinstripe babies (100% pinstripe clutches rather than the normal expected 50% pinstripe when bred to normal for pinstripes). So, pinstripe is known not to be homozygous lethal. Also, the homozygous pinstripe genotype looks just like the heterozygous pinstripe genotype which is what classifies the mutation as dominant.