Genetics question -

[Seneschal]

No, if it were dominant and the parent carrier was heterozygous spider (visible spider) bred to a normal then the spider parent would only contribute one allele, not necessarily the one necessary for a spider morph.

---S---N
N| SN NN
N| SN NN

so, 50% spider, %50 normal.

---S---N
S| SS SN
N| SN NN
50% heterozygous (but still visible spider), 25% normal, and 25% homozygous spider--theoretically identical to the het spiders, so you wouldn't be able to tell unless you bred it to a normal.

---S---S
N| SN SN
N| SN SN

so the homo spider bred to a normal would make all spiders, thus theoretically proving the spider a dominant gene and proving that particular spider parent to be homozygous for the trait.