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Human sex chromosomes evolved from autosomes. Nineteen ancestral autosomal genes persist as differentiated homologs on the X and Y chromosomes. The ages of individual X-Y gene pairs (measured by nucleotide divergence) and the locations of their X members on the X chromosome were found to be highly correlated. Age decreased in stepwise fashion from the distal long arm to the distal short arm in at least four "evolutionary strata." Human sex chromosome evolution was probably punctuated by at least four events, each suppressing X-Y recombination in one stratum, without disturbing gene order on the X chromosome. The first event, which marked the beginnings of X-Y differentiation, occurred about 240 to 320 million years ago, shortly after divergence of the mammalian and avian lineages.
The human X and Y chromosomes, like those of other animals, are thought to have evolved from an ordinary pair of autosomes (1). The pseudoautosomal regions at the termini of the X and Y chromosomes still recombine during male meiosis, ensuring X-Y nucleotide sequence identity there. Elsewhere on the X and Y chromosomes, however, X-Y recombination has been suppressed. These nonrecombining regions of the X and Y chromosomes have become highly differentiated during evolution, and only a few X-Y sequence similarities persist within them. These modem X-Y gene pairs are the remaining "fossils" where extensive sequence identity between ancestral X and Y chromosomes once existed. The recent discovery of many X-Y genes has made it possible to examine the entire group to search for patterns of human sex chromosome evolution. Thus far, the human sex chromosomes-the best characterized mammalian sex chromosomes-have been found to contain 19 X-Y gene pairs (2).
We first compared the locations of all 19 pairs of genes on the human X and Y chromosomes (Fig. 1). We determined the relative positions of the X-linked genes through radiation hybrid analysis, in many cases confirming previously published localizations (3). Map positions of the Y-linked homologs were obtained principally from the literature (4-6). On the X chromosome, most of the X-Y genes map to the short arm, where they are concentrated toward the distal end. By contrast, the X-Y genes are found as singletons or small clusters throughout the euchromatic portion of the Y chromosome. In general, the map order...