One gene, SRY (for the sex-determining region of the Y chromosome), determines sex in humans. However, many genes are involved; SRY is the critical switch, and individuals with SRY develop into males, while those without it develop into females. This mechanism of sex determination requires that SRY is active in half the population and not involved in the other half, and this is achieved by its location on a chromosome inherited by half the people and not the other half: the Y chromosome. Males produce sperm with an X chromosome or a Y chromosome in equal numbers; eggs fertilized by a Y-bearing sperm develop into males, and those fertilized by an X-bearing sperm develop into females. Thus, the primary biological role of the Y chromosome is sex determination.
Many of the other unique properties of the Y chromosome can be understood as consequences of this primary role. There can be only one copy per cell of the Y, a condition named haploidy. However, successful production of sperm (as of eggs) requires that chromosomes recombine. Autosomes, and the X in females, recombine with their homolog (the corresponding chromosome from the other parent), but a haploid chromosome like the Y needs to recombine with a different chromosome; in this case, the X. Such recombination must not disrupt the sex determination mechanism, and this is achieved by partitioning the chromosome into recombining segments located at each end of the X and Y, and a nonrecombining Y-specific segment carrying SRY in between (Figure 1). The recombining components share many properties with the autosomes and are termed pseudoautosomal. These properties include the obvious ones of their inheritance pattern and diploidy and the less obvious ones like regular gene density.
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