Shell color in crabs is determined by a polygenic inheritance system, possibly involving involving as many as 9 genes. For simplicity let’s consider the influence of 3 genes: A, B and C, where a dominant allele darkens shell color .
Crab A: aaAAaaABaABC
Crab D: aaaaAAaaBaaa
Crab B: aaAAaaBaCaaa
Crab C:aaAAaaaaaaaa