Sex-Links and the W Chromosome

Ignored by Male Researchers for a Century, We are Finally Learning About the Importance of the Female Chromosomee

Sex-Links and the W Chromosome

For years, sex-linkage in fowl has been a well-known and fairly well-understood fact. All birds have a “ZZ/ZW” sex- chromosome system. That is, males have two Z sex chromosomes in their genetic make-up, or genome, and females have one Z and one W sex chromosome in their genetic make-up, or genome.

Sex-linkage in fowl has been understood since 1910, thanks to researchers William Bateson and Reginald Punnett, who did extensive work on feather-barring and published their findings that year. They determined that a number of traits are controlled by genes that are attached directly to the Z or “male” chromosome. In many cases, this theory is correct, and has been substantiated by our current systems of gene and chromosome mapping 100 years later.

One theory about the whole picture is changing, and changing in a major way. For many years it was assumed that the W chromosome, or the “female” sex chromosome, was just a rudimentary piece of leftover or non-functional DNA. It is very small, and early researchers often missed it entirely. In any event, it was considered virtually useless. This belief persisted into fairly recent times. In fact, one textbook from a well-respected European publishing firm, printed in 1984, gave a very brief brush-off to the W-chromosome issue, dismissing it as serving “no functional purpose.”

Fast-forward just six years later. Beginning around 1990, and thereafter, research on the W-chromosome began to be done by numerous researchers, and in 1997 or 1998, research picked-up at a very rapid pace. Studies of the W female sex chromosome in organisms having a ZW system has almost become a separate field of research.

Thanks in large part to improvements in staining techniques, researchers are now able to view and study this chromosome in much more depth. Not only is the W chromosome of chickens and other related fowl being studied, but numerous other animals with a ZW chromosome genome are included in the research. (Many types of moths and butterflies are being studied. Silkworm ZW genomes also are being investigated.)

For many years, both the W sex chromosome in birds (including all poultry), and the Y sex chromosome in most mammals (including humans) were relegated to a classification of “minor chromosomes.” Both have striking similarities. It was believed, and perpetuated, that both served very minor purposes in the whole big scheme of things. Current indications and findings now suggest that this may be wrong.

With current techniques of specimen staining and microscopy, researchers are able to see at least 10 identifiable genes on the W sex chromosome of the female chicken. At least eight of these genes appear to possibly match some of the genes on the Z sex chromosome. Many genes must have a matching, or corresponding gene, on the corresponding chromosome, in the chromosomal pair, to be effective. Based on this alone, it is quite possible that the W chromosome, as well as the attached genes or DNA segments, may play a much larger role than researchers once believed.

Sex-Links and the W Chromosome
Any gene attached to one of the sex chromosomes is considered sex-linked, and the resulting trait is known as a “sex-linked” trait. In poultry and birds, it has generally been assumed, and taught, that all sex-linked traits are totally linked to one or both of the Z chromosomes. Now we have enough information to make us question at least part of that theory. In other words, are there some “helper” or modifying genes attached to that W sex-chromosome that we never suspected before?

We now know that the W sex chromosome does indeed exist as a true chromosome, and yes, there are “sex-linked” genes attached to it. We just don’t know exactly what they do at this point in time. There are a number of things that are now suspected. For example, some research indicates that fecundity, the fertility rates in various bird species, may be linked to genetic information carried on the W chromosome. Some researchers also suspect that traits of broodiness and maternal instincts might be at least partially related to genetic information associated with this chromosome. These are only a few of a continuing number of research-based hypotheses being explored in more depth.

I am keeping this article shorter than most articles that I write. I could continue, and write in some depth, about sex-linked genes, and resulting traits, attached to the Z or male chromosome. However, the very first research papers in this area were published more than 105 years ago! While much of this information is very basic, and is still used in the poultry breeding industry today, I wanted to shy away from what has been written, and rewritten so many times, and share some new information.

If you have the time and interest in the subject, I encourage you to do some research on the W chromosome. The ongoing findings are quite interesting, and may alter a few things we have held as concrete beliefs, in genetic studies.

Most living organisms are complex, including chickens and human beings. And it seems that throughout human history, males have always complained that they just don’t understand females. So I find it amusing, and somewhat ironic, that one of the least understood things in the whole big scope of the genetics picture, has been the W, or female, chromosome! I suppose it stands to reason however: Most of the researchers were males! So, next time you go out to the chicken coop and see those hens, remember they are probably not understood as well as they should be.

Originally published in the August/September 2016 issue of Backyard Poultry magazine.


Gibbs, H.L., et al., Genetic Evidence for FemaleHost-specific Races of the Common Cuckoo, Nature,2000, September 14, U.S. National Library of Medicine,National Institute of Health,

Garcia-Moreno, Jaime, and Mindell, David P.,Rooting a Phylogeny with Homologous Genes on Opposite Sex Chromosomes (gametologs): A Case Study Using Avian CHD. Oxford Journal, volume 17, issue 12, December 2000,

Nam, Kiwong and Ellgren, Hans, The Chicken (Gallus gallus) Z Chromosome Contains at Least Three Nonlinear Evolutionary Strata, Genetics, October 16, 2008, vol. 180 no. 2, 1131-1136.

Mank, Judith E., Small but Mighty: Evolutionary Dynamics of W and Y Sex-chromosomes, Chromosome Research, 2012, January; 20(1):21-33.

Dean, R., and Mank, J.E., The Role of Sex-chromosomes in Sexual Dimorphism; Discordance Between Molecular and Phenotypic Data, Journal of Evolutionary Biology, 2014.

McQueen, Heather A., Division of Developmental Biology, Roselin Institute, Clinton, Michael, Edinburgh University, Avian Dosage Compensation; Avian Sex-chromosomes: Dosage Compensation Matters, 2010.

Hutt, F.B., Ph.D., D.Sc., Genetics of the Fowl, McGraw-Hill Book Company, 1949.

Monhadam, H.K., etal., W Chromosome Expression Responds to Female-specific Selection, U.S. National Library of Medicine, National Institute of Health, May, 2002.

Romanov, M.N., Genetics of Broodiness in Poultry – A Review, Asian-Australian Journal of Animal Science, volume 14(11), November, 2001.

Ibid., et al., (Division of Integrative Biology, Roslin Institute, Edinburgh, Scotland), Genetic Control of Incubation in the Domestic Hen, Poultry Science Association, 2002.

Smeds, Linea, et al., Evolutionary Analysis of the Female-specific Avian W Chromosome, Nature Communications 6, Article no. 7330, published 04 June, 2015. (citing Smeds, et al.,) DNA Which Only Females Have, Scientific Blogging, Science 2.0, June 10,2015.

Xu, H.P., et al., The Dopamine Dz Receptor Gene Polymorphism Associated with Chicken Broodiness, Oxford Journals, Poultry Science, volume 89, issue 3, 2009;

Expression of the Chicken W Chromosome and the Evolution of Female Phenotypes, Research Councils, U.K., Gateway to Research; Department of Genetics, Evolution and Environment, University College, London, 10 January, 2016.

Leave a Reply

Your email address will not be published. Required fields are marked *