Think of the genes as fashion rules for chickens. All chickens have a pair of genes for skin color. Think of two genes as pants and socks.
Here are the fashion rules:
You can wear white pants with white socks. (Your clean original Davis roo, and Buddy Henry hens)
You can wear white pants with yellow socks. (Your 2nd gen Davis roo who carries yellow but has white legs)
You can only wear yellow pants with yellow socks. (Your splash wyandotte loaner) He shows yellow because he has yellow pants AND yellow socks.
Pants cover socks. If you have white pants with yellow socks, you don't see the socks. The hens you are trying to find have white pants and yellow socks.
When you mate them to your Splash loaner, your hens can contribute only one gene to their offspring. Randomly, some of their offspring will get their white pants, but some will get their yellow socks. It will split up fairly evenly 50/50. Dad has yellow pants and yellow socks. Since dad is all yellow, they will automatically get yellow socks from Dad.
So your chicks will be:
white pants from mom, yellow socks from Dad - white shanked yellow carriers like the ones you're trying to weed out.
yellow socks from mom, yellow socks from Dad - yellow shanked
If you get even one chick from either hen that has yellow shanks, then you know that mom has hidden yellow. But you have to split up their eggs and mark the chicks at hatch to know which ones develop the yellow legs. Don't assume that just because your
ebay Davis hen carries, that her daughter carries too. Her daughter has a 50% chance of being clean.
If the
ebay hen's daughter is clean, then she has white pants AND white socks, and her offspring in the mating with loaner roo will make chicks with white pants over yellow socks and you won't see ANY yellow. This would be great news.
Sorry if the sock/pants thing is a bit silly, but it might help make things more concrete.