In the Hawaiian happy-face spider (Theridion grallator), the allele for a patterned abdomen ($P$) is dominant over the allele for a plain yellow abdomen ($p$).

Two spiders with patterned abdomens are mated together. A proportion of the offspring in the offspring generation have plain yellow abdomens.

Explain how it is possible for these patterned parent spiders to produce offspring with plain yellow abdomens.