GMS6: Sex-Linkage

In the last article of the genetics mini-series (GMS), we discussed how parents contribute genetic material to their offspring, laying the groundwork for a discussion of sex-linkage in chickens.

​ Sex-Linkage

Genetics Mini-Series Article #6

As long as people have kept chickens, it has been prudent to know which are male and which are female.  After all, only the females lay eggs!  Generally, the sooner the sex can be determined, the happier chicken owners are.  However, unlike many species, chickens have no external sex organs: Both carry their reproductive organs within their body and transfer genetic material by means of their vent.

Bielefelder is an auto-sexing German breed developed in the 1970s.

Auto-sexing Breeds

Some breeds are auto-sexing, meaning that females and males look different as chicks.  Biological sex is indicated by color or pattern as is the case with the popular Cream Legbars as well as a number of other breeds (mostly ending in “-bar”!): Amrock, Ancobar, Barnebar, Bielefelder, Kennhuhn, Brockbar, Brussbar, Buffbar, Cambar, Cobar, Dorbar, Gold Legbar, Hambar, Niederrheiner, Norske Jaerhon, Oklabar, Polbar, Rhodebar, Welbar, Whealbar, and Wybar.  (This list is from Poultrykeeper.com’s short article on auto-sexing.  See GMS9: Autosexing Recipe for more information.)

Sex-Links

Most sex-links are hybrids, crosses between two different breeds.  They often lay very well, in part because of a phenomenon called hybrid vigor or heterosis(influenced by heterozygote advantage).  We are still trying to understand why hybrid vigor occurs, but genetic variation can be understood as genetic strength.  Because each breed has its own disease resistance profile, strengths, and weaknesses, crossing with a completely different breed can generally be seen as giving the offspring the strongest traits of both.  Recessive genetic weaknesses are masked by dominant genes for health, and thus the offspring prosper.

Hatcheries have many names for what are essentially two sex-linked crosses used to create easily-identifiable female egg layers: black sex links and red sex links.  These crosses uses sex-linked genetic traits to “label” the females before they hatch.  They also cross breeds to enable feather-sexing in day-old chicks.

Sex-Linked Traits

Black Sex-Links using Barring (B)

A Black Sex-Link hen

Barring is a dominant trait (B) that inhibits pigment to create white bars or stripes on any background color.  This gene is carried on the Z chromosome, so female chickens have only copy of this gene (from their father).  They pass their copy on to their sons but do not pass on a barring gene to their daughters.  Female chickens can either carry the barring gene (B/-) or not carry the barring gene (b/-).  Male chickens have two copies of the barring gene, one on each Z chromosome (one from each parent).  Male chickens can be homozygous dominant for barring (B/B), homozygous recessive (b/b), or heterozygous (B/b).  Because barring is dominant, even one copy will make the male’s feathering barred.  (Interestingly, heterozygous barred males have darker, bolder barring while homozygous barred males have a lighter overall color resulting from the additional pigment inhibition.  For that same reason, the effect of barring is usually less on females because they only have one copy of the gene.)

To create a sex-linked cross using the barring gene, cross a barred female (B/-) with a solid colored male (b/b) of any color.  Barring is very easy to see on black, so the cross is usually done with a black male although other colors can be used (blue, red, anything!).  Thus, F1 females are black (black sex-links) and males are barred.  Barred chicks hatch with a white dot on their head.  In this sex-linked cross, male chicks can be identified by the white spot on their head.

Red-Sex Links using Silver (S)

Red Sex-Links

Silver is a dominant coloring (S) that turns all gold areas to white.  Depending on the plumage pattern of the breed, this could mean that the silver chick is completely yellow at hatch or that the stripes in a chipmunk pattern are simply lighter.  Again, S is carried on the Z chromosome, so females have only one copy which they pass on only to their sons.  To create a sex-linked cross using the silver gene, cross a silver (S) female with a gold (s+) male.  In this sex-linked cross, female chicks will be a gold-based color, usually red.

Feather Sexing using Slow feathering (K)

Slow feathering is a dominant trait (K) that causes chicks’ feathers to grow in slowly.  It is dominant to the wild type trait, rapid feathering (k+).  (Slow feathering causes barring to appear crisp, like in Barred Rocks.  When combined with rapid feathering, the barring gene makes a messier pattern called cuckoo barring.)  When a chick is a day old, rapid-feathering chicks will have primary wing feathers that are longer than the covert feathers, and slow-feathering chicks will have short primaries the same length as their coverts.  (See Chick Sexing Techniques for full explanation.)  Slow feathering is carried on the Z chromosome, so females only have one copy.  To create a sex-linked cross using the slow-feathering gene, cross a slow-feathering female (K/-) to a rapid-feathering male (k/k).  In this sex-linked cross, female chicks will have longer primary wing feathers than male chicks.

Other Sex-Linked Traits

There are other sex-linked traits as well.  For example, many chickens have a dark pigment over the white or yellow skin of their shanks.  This pigment, called dermal melanin, can be inhibited by the dominant gene Id.  The wild type, id+, allows dermal melanin in the shanks.  The trait is carried on the Z chromosome, so females only have one copy which they pass on to their sons.  This means that a female chicken carrying the melanin inhibitor Id can pass it on to her sons, who will have light shanks, but not to her daughters who may inherit their fathers dark shanks.  You can read more about this on Brown Egg Blue Egg.

I also forgot to mention chocolate, another sex-linked inhibitor!  See “How to Breed with Chocolate” over on the Kippenjungle chicken genetics site.

A Note on Hybrids

These crosses only work in the first, or F1, generation.  Afterwards, the genes are not in place to repeat the cross using only the offspring to obtain the same results.  For example, the F1 generation resulting from the black sex-link cross consists of heterozygous barred males (B/b) and hemizygous (meaning they have only one instead of a pair – hemi, or half) unbarred females (b/-).  Thus, there are no barred females and unbarred males in the F1 generation to use to recreate the cross.  One must return to the original cross to obtain the same results.

Resources

For the best explanation, plenty of pictures of sex-linked chicks, and lists of which breeds are best to cross for each sex-link, see the Sex-Linked Information thread on Backyard Chickens Forum.