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It has become increasingly evident that gene content of the sex chromosomes is markedly different from that of the autosomes. Both sex chromosomes appear enriched for genes related to sexual differentiation and reproduction; but curiously, the human X chromosome also seems to bear a preponderance of genes linked to brain and muscle functions. In this review, we will synthesize several evolutionary theories that may account for this nonrandom assortment of genes on the sex chromosomes, including 1) asexual degeneration, 2) sexual antagonism, 3) constant selection, and 4) hemizygous exposure. Additionally, we will speculate on how the evolution of sex-chromosome gene content might have impacted on the phenotypic evolution of mammals and particularly humans. Our discussion will focus on the mammalian sex chromosomes, but will cross reference other species where appropriate., (Copyright 2004 Wiley Periodicals, Inc.)

Most eukaryotic chromosomes, akin to messy toolboxes, store jumbles of genes with diverse biological uses. The linkage of a gene to a particular chromosome therefore rarely hints strongly at that gene's function. One striking exception to this pattern of gene distribution is the human Y chromosome. Far from being random and diverse, known human Y-chromosome genes show just a few distinct expression profiles. Their relative functional conformity reflects evolutionary factors inherent to sex-specific chromosomes. [ABSTRACT FROM AUTHOR]

The X chromosome has traditionally been characterized as a conscientious sister to her derelict brother that is the Y. Beyond dutifully maintaining the family heritage, however, the X has developed its own unique identities. Now, the complete sequence of the human X allows us to appreciate its distinctiveness at an unprecedented resolution. [ABSTRACT FROM AUTHOR]