MT Ross et al. 2005. The DNA Sequence of the Human X Chromosome Nature 17 March 434, pp325-337.
"Evolutionary geneticists believe that the two mammalian sex chromosomes, X and Y, were once identical. As mammals began to diverge from their reptilian ancestors, some 300 million years ago, the proto-X and proto-Y chromosomes took on the role of determining an individual's sex. Both initially started accumulating genes from elsewhere in the genome, but over time the two chromosomes began to grow apart; the Y started to shrink and lost many of its genes." (E. Check. 2005. Genetics: The X Factor Nature 434, 266-267)
To quote a review accompanying the published sequence: "And the complete sequence of the X chromosome, published in Nature this week1 (see also News and Views, page 279), confirms that an unusually large number of its genes code for proteins important to brain function.... A woman uses only one of her two X chromosomes in each cell, so if one of her X chromosomes has a defective gene, only some of her cells will suffer. But men have only one X, so any defective brain genes from that chromosome are invariably expressed."
( Gee...from this, one might ponder the possibility that originally females began to reproduce by parthenogenesis, in which the egg divides and yields a fertilized zygote with a complete set of chromosomes...? ....snickers. )
