The human Y, that is. The Science Creative Quarterly has a very thorough (ie, make sure you have some time to spare) review of the mammalian Y chromosome (focusing on the human Y). The article covers the origin and evolution of the mammalian Y and what the degeneration of the Y means for the future of human male fertility and sex determination. I should point out that the mammalian Y chromosome is an anomaly in origin and sex determination. In fact, every single sex determination system and sex chromosome system that I know of differs from all of the others in some manner. It looks like I'm going to have to write an entry on the evolutionary genetics of sex determination in everything other than mammals (because the SCQ has already covered the furry guys).
Added 20March2006: Of course, eutherian sex chromosomes are boring compared to monotremes. Check out the crazy stuff going on in the platypus.
There is also an excellent review of mammlian sex chromsomes in the current issue (10th march) of 'cell' called 'Sex Chromosome Specialization and Degeneration in Mammals' by Jennifer A. Marshall Graves.
It can be downloaded for free by clicking this link:
http://www.agrf.org.au/LinkClick.aspx?link=graves+Cell+final+100306.pdf…
('save as' doesn't work for this link)
"In fact, every single sex determination system and sex chromosome system that I know of differs from all of the others in some manner."
But they are also very similar in many ways. Sex determination systems are usually cascades of regulatory genes terminating in a final switch that makes an organism male or female. Downstream elements of the cascade are often highly conserved, but the upstream parts can vary wildly, even within species. A possible reason is that the upstream parts are vulnerable to being "hijacked" by "selfish" genetic elements that bias the sex ratio in order to gain an "unfair" transmission advantage.
Despite the diversity of sex-determining systems, Haldane's rule holds across a variety of taxa (hybrids that are the sex with two different sex chromosomes - the equivalent of X and Y - tend to have lower fitness than hybrids of the other sex). Does this suggests an underlying similarity, at least at some level?
Good points, guys, but I was actually thinking more along the lines of the different chromosomal types of sex determination (rather than physiological/biochemical). For instance, how XY systems differ between mammals and Drosophila, as do the origins of the mammalian Y and Drosophila Ys (yes, Drosophila Y chromosomes are not homologous). Doris Bachtrog's done some cool stuff on Y degeneration in Drosophila that I could probably write about.
There are also some interesting anecdotes like how in many taxa sex is determined environmentally. Or, some organisms can change sex in the middle of their life depending on population dynamics. Or, how in birds the heterogametic sex are females. Mostly, I wanted to focus on the diversity of sex determination at the chromosomal level.