Sex-Link Chicken Genetics

A sex-linked chicken, quite simply, is a chicken whose sex is visible to the naked eye immediately after hatching. Usually male and female sex-linked chickens have markedly different colors or feather patterns. The reason that this is desired is that most people want hens for egg-laying purposes, and roosters not only are not desired, but are even prohibited by local ordinance in some communities because of their loud crowing.

Sex-linked Chickens are Hybrids

In the first place, both red sex-link, also called Red Stars, and black sex-link chickens, also called Black Stars, are, by definition, hybrids. That means they are not true breeds unto themselves, but are the result of crossing a rooster of one breed and a hen of another breed in order to get off-spring with certain characteristics.They do not breed true in subsequent generations.

Sex-linked Wing Feathering Rates

There are several kinds of sex-linked chickens that are popularly sold by the big hatcheries. Red sex-links and black sex-links are the most popular. There is also a sex-linked gene that allows the wing feathers of male and female chicks to grow at different rates that can be used to determine whether day old chicks are male or female. The wing feathering genes can be used in certain breeds of pure-bred chickens for those that want their flocks to breed true, generation after generation. For a more in-depth catalog of wing feathering rate and other genes, the Sellers family maintains a list of Chicken Genes of Common Interest.

Sex-linked Genes

In chickens, the female has one gene for sex and the male has two. The male gives a sex-link gene to all of its offspring, while a hen may give either one or none. Off-spring who received two sex genes (one from each parent) are males, while those receiving only a single sex link gene (from the rooster), are females. Therefore, any baby chick that receives a dominant color gene from its mother when crossed with a rooster whose breed has only recessive sex-linked color genes will exhibit the color dictated by the mother's dominant gene and will be a male. Others will exhibit color like the parent rooster and will have a single recessive color gene.

Sex-link Chicken Parent Breeds

Red sex-link chickens are a cross between a predominantly red rooster breed such as the New Hampshire Red which carries two recessive gold color genes, designated s+, and a hen which carries the Dominant Silver gene, designated S, such as a Delaware White. These are not the only two breeds used to create the red sex-linked hybrid; any that carry the appropriate sex-linked genes for color can be used. Any baby red sex-link chick who received the Silver gene and thus presents a yellow down and grows white feathers is necessarily a male. Female red sex-link chicks will have a reddish down and will grow red feathers because they do not carry a copy of the dominant Silver gene. As red sex-link roosters attain maturity, they will typically show some red patches on their backs, but will remain predominantly white. The same scenario holds for black sex-linked chickens. Except in this case, a New Hampshire Red or Rhode Island Red rooster is crossed with a Barred Rock hen. The male black sex-linked chicks, will be distinguishable by a white spot on the top of their heads. The males may also show a light barring pattern, while the females will not.

Hybrid Vigor in Sex-link Chickens

Hybrid chicken breeds like the sex-linked varieties often demonstrate hybrid vigor or heterosis. Hybrid vigor is defined by Gardner and Snustad in Principles of Genetics as "Unusual growth, strength, and health of heterozygous hybrids from two less vigorous homozygous parents." In other words, hybrid offspring may be bigger, stronger, and lay more eggs than the typical breed of either of the parents. This is another reason that hybrid breeds like the sex-linked chicken are widely used for egg production.

Sex-linked Chickens Do Not Breed True

The problem with both black and red sex-link chickens is that after the initial generation of hybrids, all the babies will no longer be sex-linked. The males are now heterozygotes, meaning they each have one Dominant Silver gene and one gold (red color) gene. Thus they will give fifty percent of the females a Silver gene and 50% a gold gene. The males will get a recessive red gene from the mother and either a Dominant Silver or a second recessive gold gene from the father. Therefore, both male and female chickens from the second generation of red-sex links will be half red chickens and half white chickens. The images attached to this article show Punnett Squares for first and second generation Red-sex Link chickens as well as a conversion of genotype to phenotype (appearance).

Third Generation of Sex-Links May Show Sex-Linked Color Again

Looking at the second generation Punnett Square, however, we can see that red males of this generation are once again homozygotes for recessive red color genes. Similarly, White females of this generation possess only the dominant Silver gene. Using these portions of the 2^nd generation, one could again achieve sex-linked coloration in third generation chicks using just this portion of the second generation population. Other traits would not fall in line with the sex-linked coloring, so hybrid vigor would not necessarily be present.

Sources:

Sellers family. Chicken genes of common interest. Retrieved from sellers.kippenjungle.nl/page3.html on March 2nd, 2010.

Gardner, Eldon J., and Snustad, D. Peter. 1984. Principles of Genetics, Seventh Edition. John Wiley & Sons, Inc., New York.