The X - a sexy chromosome

There is new and convincing evidence((1)) that the mammalian X chromosome, as well as the Y chromosome, contains an atypically high proportion of gene s involved In sex and reproduction (SRR genes). Here we consider alternativ e explanations for this concentration. One possibility is that a particular ly well-endowed autosome was "chosen" for a career as a sex chromosome. Alt ernatively, the high concentration of SRR genes may have resulted from the accumulation of these genes on the X after the degradation of the Y, either by transposition of autosomal SRR genes to a "selfish X", or by acquisitio n of SRR functions by widely expressed genes on the X. We suggest experimen ts to distinguish these possibilities, and speculate on the implications of gathering evidence that genes with other functions, too, are not distribut ed uniformly over the genome. (C) 2001 John Wiley & Sons, Inc.