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| Sex and the Y Chromsome |
Except for the sex chromosomes (X and Y), all humans have the same set of chromosomes. The karyotype of a human male is 46XY (46 chromosomes, including one X and one Y), and that of a female is 46XX (46 chromosomes, including two X chromosomes.) In mammals as a whole, the presence or absence of the Y chromosome usually determines sex. Individuals with an X chromosome but no Y (45XO) are female (Turner's syndrome); individuals with two X chromosomes and a Y chromosome (47XXY) are male (Kleinfelter's syndrome).
The Y chromosome is considerably smaller than the X chromosome and has a much lower density of genes. In fact, the Y has often been called a "genetic junkyard." But there are a few rubies among the rubbish of that genetic junkyard: the Y chromosome contains the genes are essential for male fertility and other male characteristics.
Why does the Y chromosome have so few functional genes? Evolutionary biologists are still debating the details but they agree that the lack of recombination explains the paucity of functional genes on the Y. Unlike the twenty-two pairs of autosomes, there is no recombination between the X and most of the Y chromosome. Genes on the part of the Y chromosome that does not recombine will be passed from father to son, down a paternal lineage, and will never be present in females. The lack of recombination weakens the effectiveness of natural selection to weed out bad variants and select for good ones. Over many millions of years mutations and random genetic drift erode the Y chromosome, turning it into a genetic junkyard. In contrast, genes on the X are present in both males and females; X chromosomes, like autosomes, recombine in production of female gametes.
About five percent of the Y chromosome does recombine with the X. This region, at the tips of the chromosomes, is called the pseudoautosomal region because in it the X and Y chromosomes behave as autosomes (Fig. 1). The pseudoautosomal region is more gene-rich than the rest of the Y chromosome. Several of the genes on the pseudoautosomal region of the Y have counterparts on X, reflecting a common evolutionary ancestor. The genes required for male fertility are found in the non-recombining regions of the Y, and are not present on X.
Researchers in David Page's lab have shown that one-quarter of the Y chromosome consists of eight families of nearly identical nucleotide sequences, and includes duplicate copies of important genes. Because these regions are arranged in palindrome fashion, they provide a mechanism for a kind of internal recombination between the similar genes on the same chromosome. This process, called gene conversion, aids in the detection and repair of gene mutations in this part of the Y chromosome.2