By Doug Ottinger – Did you know that even if you have only purebred birds in your flock, they might be sex-links? Wait, don’t panic! Your birds are still purebred! The hatchery did not pull a “fast one” and sell you some sort of “Franken-chicken,” or a half-breed posing as a purebred. The terms “sex-link” and “sex-linkage” are often misunderstood, and sometimes have unnecessarily bad connotations to poultry keepers.
In the purest sense of the term, a sex-linked trait is simply one that is carried on a gene attached, or linked, to either the male or the female sex chromosome. Hence the term, “sex-link.” Sex-link chickens are the result of crossing birds, which have certain sex-linked traits so that the offspring can be differentiated sexually at time of hatching. In the case of chickens and other birds, these are traits that are carried on the male or Z chromosome. The female chromosome in birds, the W chromosome, is very small and has genes attached, but even after one hundred-plus years of intensive genetic research in poultry, we still have very little comprehensive knowledge about its workings or what traits the attached genes possess.
In birds, males have two Z sex chromosomes, and females have one Z and one W sex chromosome. Probably the best-known sex-linked trait in poultry is black and white barring of the feathers, as is found in the Barred Rock and Dominique chicken breeds. Barring is a dominant trait, carried by a gene on the male, or Z sex chromosome. Being dominant, only one gene is necessary to give the trait to the next generation. If a hen with barred feathers is mated with a red-colored rooster, such as a Rhode Island Red or New Hampshire Red, the male offspring will be barred, and the females will be a black color with a few blotches of red, when mature. These females are known by such trade names as Black Stars and Black Sex-Links and are generally very good laying hens. Immediately after hatching the males and females can be separated because of how they look. The cockerels will be black with a large yellow spot on the head. Their legs will also be yellow in color. The pullets will be all black, with dark coloring on the legs and beak. There are numerous sex-linked traits used in the poultry industry today so the chicks are easy to sex at hatching.
Another set of sex-linked traits are the plumage colors silver and gold. In the simplest of terms, silver is a genetic form of white, or white with (usually) some black pattern on it, that is carried on the male sex-chromosome. It is dominant to the gold plumage color. Examples of silver breeds include Delawares, Silver-Laced Wyandottes, Light Sussex, and Rhode Island Whites. Examples of gold breeds include Rhode Island Reds, New Hampshire Reds, many of the buff-colored breeds, and Brown Leghorns. Older information from 50 to 80 years ago mentions White Rocks as also having sex-linked silver plumage. These strains of White Rocks are hard to find and are almost non-existent today.
Silver hens crossed with gold roosters produce male chicks that are lighter yellow in color and female chicks that are a darker yellow, or light brown. These types of crosses are very popular for egg-laying strains. They are generally good layers with calm dispositions. Trade names include Cinnamon Queens, Red Stars, Sil-Go-Links, and Golden Comets. The female offspring have red feathers at maturity, and the males have silver or white feathers.
How about heritage chicken breeds that are also sex-links? Wow! Does that ever sound like an oxymoron! How can a purebred heritage breed also be a sex-link? Most often, we think of a sex-link as the result of breeding two separate breeds, that result in offspring that can be differentiated by color, at time of hatching.
However, there is one sex-linked trait, slow feathering, which is dominant and carried on the Z chromosome, that can be used in breeding even purebred and heritage chicken breeds, and will allow sex differentiation in the new hatchlings. The terms slow feathering and rapid feathering have more to do with how the chicks look at hatching, than long-term development (there will be some slight differences in speed of feather development, but not usually a great amount).
For many years, vent sexing was the industry standard for determining the sex of most newly hatched, purebred chicks. Chick sexers who were trained, and had very sharp eyesight, were in demand. They could differentiate chicks by extremely slight differences in the small, dermal papilla of the vent. A good sexer could determine the sex of the chicks at a surprisingly fast pace, and with about 95 percent accuracy. It was a field that was in demand and the sexers often worked on contract, with one or more hatcheries, rotating between hatcheries on hatch days. Today vent sexing has become an all but lost art, and hatcheries have had to develop other ways to determine the sexes of the newly hatched babies.
Some poultry breeds are known to hatch with a well-developed start to the primary feathers on the tips of the wings. This trait is known as rapid feathering. Other breeds are known to hatch with primaries that are not as well-developed. This trait is known as slow feathering. It was correctly determined, many years ago, that these traits were sex-linked and carried on the male or Z chromosome. It was also noticed, that even within the breeds, there was a fair amount of differentiation in primary feather development at the time of hatching. Experiments and reciprocal crosses showed that these variations were also sex-linked. They found that when they crossed a slow feathered female with a rapid-feathered male, the female chicks hatched with rapid feathering and the male chicks hatched with slow feathering. The chicks could be immediately sexed after hatching, with a high degree of accuracy.
Poultry folks began finding out that they could develop both rapid-feathering lines, as well as slow-feathering lines of the same breeds, without sacrificing breed standards. Fast forward to where we are today. According to Jeff Smith from Cackle Hatchery and Tony Halstad from Hoover’s Hatchery, the hatchery industry today sexes almost all its main breeds, as well as heritage chicken breeds, by this method. Hatcheries and breeders keep both slow feather and rapid feather strains of the same breeds going and cross-breed these to get baby chicks that can be feather or wing sexed at time of hatching. This method is also used for many hybrid sex-link crosses, including Cornish Cross chickens, various strains of multi-colored, slower growing meat birds, and strains of laying chickens that are not sexed by color.
Since this trait is sexed-linked and is used in the breeding of pure and heritage breeds, the resulting purebred offspring, in the truest sense of the term, are also sex-links! Now the riddle is solved how pure breed birds can also be sex-links!
There are other numerous sex-linked traits that can be used for breeding and sex determination in fowl. The wild-type pattern — those baby birds that have the little chipmunk stripes on their back — is one of these. Color patterns of many breeds are sex-linked, or partially sex-linked, and are far too numerous to mention here. Even brown eye color in chickens is because of a gene on the Z chromosome!
Whether you raise purebred birds or just have some little mixed-breeds as backyard pets, your birds probably have sex-linked genes somewhere in their genome or genetic make-up. No matter what group they fall into, chickens are truly fascinating little creatures. What sex-linked oddities interest you?