Fair warning, I know hardly anything about chicken genetics, and everything I’ve learned is from looking at these forums since the beginning of summer.
However, I think yellow skin is recessive, so both your rooster and the mother hen must carry at least one gene for yellow skin, though it may not show since white skin is dominant. The chick received two yellow skin genes, thus it has yellow legs. If you know for sure your cuckoo chick came from a Cuckoo Marans egg, then I would say the mother is not a pure Marans.
I’d wait until a genetics expert pops in to confirm this, or sets me straight if I’m incorrect, but that is my understanding of the matter. I only hope that I’m being helpful and not just making things more confusing for you!
