Chickens / Genetics Mini-Series

GMS Supplement #2: Skin Color Punnett Squares and Test Crossing

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.

This Basque Hen cockerel has yellow legs. He is homozygous recessive, ww.

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.

One of the Basque Hens above has yellow legs while the other has white. It is likely that the white-legged hen carries the yellow gene.

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.)

Chart 1

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).

Chart 2

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.

Chart 3

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.

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8 thoughts on “GMS Supplement #2: Skin Color Punnett Squares and Test Crossing

  1. Pingback: GMS10: Skin and Shank Color « Scratch Cradle

    • There are a number of breeds for which the standard of perfection requires white skin. Sometimes, especially in the development of new colors, yellow-skinned birds are introduced, and then the skin color needs to be bred back out. An example of this would be the Silver Ameraucana, which requires slate legs, based on white, but sometimes shows willow, resulting from yellow skin. Other times, people just want to create a new line or breed based upon a drawing in their head! Generally, English breeds have white skin because they prefer a white carcass, while American breeds have yellow skin because Americans prefer a yellow carcass.

      • So it is just a color thing, for me here in Costa Rica i’m hoping to get good layers and fair meat birds out of the local “general household’ flock. I did like the way you choose you birds for weight and such, i will be using that as a guide line when i choose the 10 hens and 1 roo out of the 30 chicks I have. Thanks for the fast responce.

        Art

      • Yup, just a color thing. I was just explaining the genetics bit for those who are interested. Both of the breeds I am trying to work with actually have yellow legs, so I am interested in keeping the yellow in! Thanks, Art!

  2. hi there just a quick question as i cant get my head round the square but if i breed a rooster who has slate (blueish black) with hens who have yellow legs will the yellow legs be passed on to male offspring.thanks

    • The hens will each carry two recessive genes for yellow. Yellow skin is not sex-linked, so both male and female offspring have an equal chance of inheriting the yellow skin recessive gene. Unless the rooster is carrying a recessive gene for yellow skin which you do not see, then none of the offspring will show yellow skin, but the next generation might.

  3. Your knowledge of chicken genetics is just amazing. The squares are hard for me to grasp as well, so I hope you can help; Here’s my questions: if you test both a Black Copper Marans Birchen rooster and BCM hen for Wheaten (and they both test out as non Wheaten) is it possible for the pair (or the hen) to still carry recessive genes for yellow skin that show up later on? Wouldn’t the Wheaten test ferret out any yellow skin gene?

    One more question: If you mate a Birchen and split Wheaten, (in the odds) you get some young who are Wheaten, and some who are pure Birchen. Does this also hold true for Birchen crossed with split extended black as well- some young being extended black, but, also some who are pure Birchen? Is there a way to test the progeny for extended black, as there is the test to check for Wheaten/ Birchen?

    Many thanks for your time and knowledge.

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