Bumblebee x Cinnamon

[OhhWatALoser]

You need to have a dominant gene in order not to hit normals.
Co-doms (or incomplete dom) will throw normals because they're not dominant traits. But more genes you have in play, the chances of normals decrease (theoretically).

if it is homozygous, it will not throw normals, being dominant or incom/co dom has nothing to do with it.

there is also situations where there are two mutant alleles sitting on the same locus, so you get one mutant gene or the other.

to answer your question OP, this is why.

Bumblebee x Cinnamon Punnet Square. Put down each combination of genes each parent can give, the bumble bee can give pastel and spider, just pastel, just spider, and none. The cinny can give a cinny gene and none. As you can see there is a chance for both parents to give no mutant genes.

	PSc	pSc	Psc	psc
psC	PpSsCc (Pewterbee)	ppSsCc (Cinnabee)	PpssCc (Pewter)	ppssCc (Cinny)
psc	PpSscc (Bumblebee)	ppSscc (Spider)	Ppsscc (Pastel)	ppsscc (Normal)