The other day I mentioned an important recent paper on the sequencing and evolutionary history of the human X chromosome.
The human genome -- our complete set of genes -- comes packaged in 23 pairs of chromosomes. The members of each pair are identical, except for the X and Y. The X is average in size for a chromosome. The Y is a shriveled little thing, smaller than any other chromosome.
The cells in a woman's body contain two X's. The cells in a man's body contain an X and a Y -- and thus are we divided into sexes. A woman always contributes an X to an embryo; a man can contribute an X or a Y, depending on which sperm wins the race to the egg.
Crucially, the Y bears a gene called Sry, which makes a protein that switches on the Sox9 gene, which switches on the MIS gene. MIS makes testicles in a developing embryo, and testicles make boys by releasing hormones.
The diminutive but scrappy Y has long engaged the larger and overbearing X in evolutionary strife. The X and Y may originally have evolved from a matching pair of non-sex chromosomes. But once they got involved in sex determination, all hell broke loose. As the biologist Matt Ridley puts it, "The two chromosomes no longer have each other's interests at heart."
The X chromosomes spend two-thirds of their time in females and only one-third in males. "Therefore," say Ridley, "the X chromosome is three times as likely to evolve the ability to take pot shots at the Y as the Y is to evolve the ability to take pot shots at the X." The result is that the Y has been reduced to a mere stump of a thing -- although bearing the all-important Sry gene.