I wrote an explanation of skin and shank colors for GMS10: Skin and Shank Colors. I had so much material to cover that I did not include some basic Punnett Squares showing how white skin’s dominance over yellow skin plays out. You’ll want to go back and read GMS10 before proceeding if you haven’t seen it before. You’ll also want to know what is meant by simple dominance, homozygous, and heterozygous, all of which can be found in GMS1.
White skin (W) shows simple dominance over yellow skin (w) and is not a sex-linked trait. Skin color affects beak color and is the epidermal layer on the shanks. Yellow skin contains yellow pigment in the form of carotenoids from the birds diet, and white skin does not.
Because white skin is dominant, a chicken who is homozygous for white skin (WW) will only have offspring with white skin, even if paired with a bird who is homozygous for yellow skin (ww). A chicken must be homozygous for yellow skin to show the trait. (In more technical terms, only a ww genotype shows the yellow-skinned phenotype.)
However, like the offspring of the cross above, you can have a white-skinned bird who is carrying a recessive gene for yellow skin (Ww). If paired with a bird who is homozygous for yellow skin (ww), approximately 50% of their offspring will be homozygous recessive and show yellow skin (ww), and 50% will be heterozygous and show white skin (Ww).
If you cross two heterozygotes (Ww with Ww), then 50% will be heterozygotes themselves, showing white skin but carrying the yellow skin gene (Ww). Twenty-five percent will be homozygous for white skin, showing white and carrying white only (WW). The final 25% will be yellow-skinned, homozygous recessive (ww). This is the only way to get yellow-skinned offspring from white-skinned parents. The parents must both be carrying the recessive gene.
It is important to note that a yellow-skinned bird can only have one genotype: ww. However, a white-skinned bird could be WW or Ww depending on its parentage. A yellow-skinned gene, or any other recessive gene, can continue to pop up throughout a breeding program seemingly randomly because heterozygotes carrying-but-not-showing this gene are accidentally retained in the breeding program.
If you are trying to eliminate yellow skin or any other recessive trait from your breeding program, the only way to do this is to test cross all of your breeders. (Test crossing was also discussed in GMS1.) To do this, you need a bird (probably one of each sex if you must test everyone) who shows the recessive trait. We will continue using the example of yellow-skin.
Put the white-skinned bird who might be WW or Ww with the yellow-skinned bird (ww). If 50% of the offspring have yellow legs, as in Chart 2 above, then the white-skinned bird was heterozgous and is carrying yellow. If you are trying to eliminate this trait, do not use this bird as a breeder. However, if all of the offspring have white legs as in Chart 1, the white-skinned parent was homozygous for white skin, WW. You then retain this parent bird as a breeder, but must eliminate all of the offspring who are surely heterozygous.