I am not familiar with the genetics of the granite trait. In the case of "classical dominant" traits, homozygous and heterozygous animals would be phenotypically identical (express the trait)but 100% of the offspring from the homozygous animal would express the trait, whereas only 50% of the offspring of the heterozygous animal would express the trait.
In the case of spiders it is my understanding that of the few spider to spider breedings, there have not as yet been any documented cases of offspring that throw 100% spiders. For this reason it is suspected that some other "non-classical" genetic explanation. Genetics "outside of sweet peas" does not usually function according to classical mendelein genetic patterns. Vertebrate genetics is much more diverse and complicated.
Good Luck
Grant

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