Creating an Autosexing Breed
Genetics Mini-Series Article #9
In the article on sex-linkage, we briefly looked at autosexing breeds. In these breeds, females and males look different as chicks. Autosexing is based upon the barring gene. One dose of the barring gene lightens a chick’s down slightly. Two doses lighten the down considerably. Using the barring gene in combination with certain background colors (see Breeding for Two Traits), autosexing breeds are created from a cross between a barred breed and a wild type or brown breed.Males can have zero, one, or two doses of B (b/b, B/b, or B/B) although females can only have one dose or none (B/-, b/-). When a male is homozygous for the barring gene, his coloring as a chick is greatly lightened. A female from the same line would be hemizygous and thus darker than the males.
To make this difference visible, you need to pair the barring gene with a background color which easily shows the difference. This difference is difficult to see on the yellow of a wheaten-based (eWh)chick and does little to lighten the black of an extended black (E) or birchen (ER) based chick.
“Autosexing. As pointed out by Silverudd (1974), sex-linked barring (B) could be called the autosexing gene, since it has been the key ingredient in all true autosexing stocks. Sex differentiation is accomplished through the well-established dosage effect of B. On appropriate genetic backgrounds, homozygous barring in males lightens down color appreciably more than does the hemizygous condition in females. This B-associated difference is apparent on a wide variety of phenotypes, but is most obvious on a striped down background.” – R. D. Crawford
For this reason, autosexing breeds are generally built upon the chipmunk-like coloring of wild type (e+), Brown (eb), or Buttercup (ebc) chick down. However, New Hampshires have been used to create the Hambar and Buff Rocks have been used to create the Buffbar. The key to all of these breeds is selection over generations, emphasizing the differences between the sexes. To see what this looks like, check out this person’s breeding project.
There is a general formula which you can use to create your own autosexing breed. To be perfectly honest, I do not know why you begin with a colored male and a barred female rather than the reverse. If you began with a barred male and colored females, F1 females would be barred, and if you bred them back to their sire, all F2 offspring would be barred. I must assume that this indirect approach helps to create a more differentiated appearance in the offspring. So here is the given formula:
Begin with your e+, eb, or ebcmale and barred females. Select breeds which emphasize the traits you desire such as egg color, body shape, crest, etc. Cross the male to the barred hens to create your F1 generation. The females will give their barring gene to their sons, who will be heterozygous, and their daughters will be unbarred. Retain the F1 males. Cross the F1 heterozygous-barred males to a female of your wild-type/brown breed.
The F2 generation will consist of heterozygous and hemizygous cockerels and pullets respectively (B/b, B/-) as well as unbarred cockerels and pullets. Cross the F2 barred offspring with each other.
Half of the resulting females will be barred B/-. These will be retained in the breeding program. The unbarred females (b/-) are not needed. All of the males will be barred: Half will be homozygous barred and half will be heterozygous. You will likely need to test cross to determine which are homozygous to keep only those. However, they should be lighter than the heterozygous males as chicks, and even noticeably as adults, and so you may be able to identify them be careful observation and record keeping.However, you are not done! Over the next few generations, select the females which look most obviously dark and males which look most obviously light. By repeatedly selecting for those which are most clearly defined, you will enhance the difference in appearance in your line.
And, before you know it, we’ll have something crazy like olive-egg-laying, crested, feather-legged, barred-blue, autosexing bantams!
Explore autosexing breeds at http://www.autosexing.co.uk/!