There are two ways a leg is colored. The skin on the outside, and then the deeper skin tissue can also be colored.
White and Yellow are on the outside. White is dominant and can hide Yellow genes.
Any whiteshanked siblings of chicks with yellow legs can carry yellow into your next generation.
In order for a chick to show yellow legs, BOTH parents had to carry the yellow. In that pairing, one in four will be yellow, two will carry, and one will be pure white.
The dermal melanin under the skin gives a bluish color.
White + dermal melanin gives us slate or blue legs
Yellow+ dermal melanin gives greenish or Willow legs.
There is a gene that inhibits dermal melanin in birds that would otherwise have dark legs. (our lightshanked BCM)
It is a sex-linked gene. Roos can have two, Hens only one. That is why BCM hens can have darker shanks than their male counterparts. Two copies of the ID gene is like barring, or blue, it lightens more than one copy.
Don't want to open a can of worms with this one, since I know the French and proposed U.S. standard are different on shank color.
But if you have a hen with dark shanks, because it is sex-linked, most likely her father was also dark shanked, or at least only carried one copy.
Sex linked genes are always passed from Father to daughter.
Seems like it should be pretty easy to breed for light shanks, or at least maintain them. Locating the birds with the missing ID genes shouldn't be that hard.
Most hens with yellow legs will gradually lose the yellow as they near their molt. They can appear almost white. But after molt they are usually once again yellow.
