The Barring Gene
In chickens there are many different genes. One of the most common genes is the barring gene. The barring gene is a gene that is commonly used to create sex-linked hybrids and also is in auto-sexing, barred, and cuckoo varieties of chickens.
First I would like to explain the barring gene. The barring gene is a dominant gene, so if a bird has it, it will be expressed, the barring gene is also sex linked so it is based on the same alleles that determine the gender. Being on the gender determining alleles makes it so that the male carries two barring genes while the female only carries one. This makes it so that when you cross pure males and females together you will get all pure offspring, but if you cross a self colored male over a barred female you will get split males and black females, and if you cross a barred male over a self colored female you will get all split males and barred females. If you cross a split male over a self colored female, half of the males will be self colored and half will be split also half the females will be self colored and the other half will be barred.
Next I would like to explain how black sex-links work using the barring gene. When making sex-links what is done is you cross a non barred male, usually a Rhode Island Red or Production Red, over a barred female; the offspring of that cross will be sexable out of the incubator because the males will have a white splotch on their heads while the females will be black with some red on their faces.
Now I am going to explain how to create an auto sexing/ crele chickens using the barring gene. Crele chickens are always based on a patterned bird, whether its Black Breasted Red, Partridge or Wheaten, and it also has the barring gene along with it. If you start with a patterned male and a barred female it will take 4 generations to create crele. The first generation you will cross the patterned male over barred females, you will cull all of the females.
The second generation you will cross the males from the first cross with patterned females, this generation you will get a lot of different colored birds but you only keep the patterend barred ones. The third generation you will cross the patterened birds that you kept from the previous crossing and you will cull all of the birds that look the crele color. The fourth generation you cross the crele with the crele and you should be getting true breeding birds from there.
Finally I would like to explain the difference between barred and cuckoo coloration. Both barred and cuckoo have the barring gene, the difference is that barred birds have a slow feathering gene that lets them get the more crisp barring, whereas the cuckoo birds do not which is why they are more blurred.