FAQ: X and Y or Z and W chromosomes?
In aviculture societies in Europe, the sex-chromosomes in birds was, for long years, referred to as X and Y, just as they do in mammals / humans. That was because, most of the articles / books, those days, used X and Y as sex-determining system in birds.
When I became involved in aviculture in 1993, I explained to judges / breeders that in science they used Z and W instead of X and Y. Trust me, nobody believed me. My first article on that subject was rejected by the editor. “Too scientific” and “not interesting for breeders” was his comments. Another editor replaced in my articles, systematically, before publishing, Z and W with Y and X.
I know it sounds strange now, but you have to watch it in the spirit of the times. Lots of people, really passionate by breeding birds, but their biggest concerns were keeping these birds alive and learn how to breed with them. Mutations were popular, but the inheritance was less important. Most of them were already happy if they understood what sex linked and autosomal was and important: they learned to refer to the sex chromosomes as X and Y, and no one could convince them of anything else (and certainly not a novice breeder like me…). Homines quod volunt credunt. LOL.
Only a few authors used correctly Z and W. But because of the language barrier (there are + 90 different languages in Europe), almost nobody in Europe was aware of it, or ….(in most cases) they considered it as ‘not correct’. They did not realise that the use of ZZ and ZW to refer to the sex chromosomes was definitely not new. In the 1960’s scholars already referred to the sex chromosomes as ZZ for a male and ZW for a female . Unfortunately, these publications were rather rare and unknow in aviculture. At the turn of the 21th century other scholars came up with more evidence for the different origin of sex chromosomes in birds and mammals –. It was clear that the avian sex chromosomes evolved from a different ancestral pair of autosomes than the X and Y chromosomes in mammals. But even then, it was not accepted / important in aviculture.
When I wrote my first book in 2004, I was ‘strongly advised’ to use X and Y (I had to keep things simple, it had to be understandable for the novice breeder etc….). Believe me, I really felt uncomfortable with it, so …. I mentioned in the same chapter, that scientifically it was Z and W [5, p. 101].
In the mean while I was still trying to convince others to refer to sex chromosomes as ZW and not XY. But no luck. That period we were also working on the International naming system, so it was important to do it step by step, because most aviculturists were (are??) very conservative.
More and more scientific publications on the avian sex chromosomes became available  and we had extra sources to refer to, but even then, most organizations refused the use of Z and W. So we decided within MUTAVI and Ornitho-Genetics VZW that we waited long enough. I wrote an article on this subject in the BVA magazine and announced that from then on, in BVA International, we should use Z and W to refer to the sex-chromosomes in birds. I can assure you that not everybody was happy with it. ?
Now, almost twelve years later, it is common in Agapornis species to refer to the sex chromosomes as ZZ and ZW, but we have to admit, not everybody is on the same track. We know that it is important to stick to one system and internationally bird sex chromosomes are referred to as ZW and not XY. But believe me, after 25 years in aviculture, I have learnt one thing: …to be patient.
But there is good news: younger breeders become involved in aviculture, they are more educated and open for these changes. We have every confidence in their ability to succeed.
Just my thoughts…
 S. W. Soukup, “Sex chromosomes and sex-linked genes. By S. Ohno. Springer-Verlag, Berlin, Heidelberg, New York. 192 pp. 1967”, Teratology, vol. 4, nr. 1, pp. 111–111, feb. 1971.
 I. Onsman, “Sex-chromosomes in Birds and Mammals; Differences and Similarities”, 1998. [Online]. Beschikbaar op: www.mutavi.info.
 J. A. M. Graves en S. Shetty, “Sex from W to Z: evolution of vertebrate sex chromosomes and sex determining genes”, J. Exp. Zool., vol. 290, nr. 5, pp. 449–462, 2001.
 J. A. M. Graves en R. J. W. O’Neill, “Sex chromosome evolution and Haldane’s rule”, J. Hered., vol. 88, nr. 5, p. 358, 1997.
 D. Van den Abeele, Lovebirds, owners manual and reference guide, 1ste dr. Over dieren, 2005.
 R. Stiglec, T. Ezaz, en J. A. M. Graves, “A new look at the evolution of avian sex chromosomes”, Cytogenet. Genome Res., vol. 117, nr. 1–4, pp. 103–109, 2007.