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Figure 1. Schematic representation of the sex chromosomes of a female and male. The active X chromosome is in orange, the inactive X chromosome in purple, and the Y chromsome in green. In females, either the maternal (mat) or paternal (pat) X chromosome is inactivated in any given cell.

recombining exchanging genetic material germ cells cells creating eggs or sperm

Role of the X Chromosome in Sex Differentiation

The so-called sex chromosomes differentiate the sexes: females are XX and males XY, which is the basis for the development of a fetus into a girl or a boy (Figure 1). All other chromosomes (called autosomes) are present in two copies in both males and females. It is the presence of a Y chromosome that determines the male sex of a baby, because the Y carries a gene that induces undifferentiated gonads to turn into testes in the fetus. The number of X chromosomes does not change the sex of a baby. Indeed, people with a single X chromosome and no Y chromosome are females with Turner syndrome, a rare genetic disorder characterized by short stature and infertility. Conversely, people who have two X chromosomes and a Y chromosome are males with Klinefelter's syndrome, which includes tall stature and infertility.

Sex Chromosome Evolution

Present-day sex chromosomes look very different from each other: The X chromosome comprises about 5 percent of the human genome, and contains about 2,000 genes, while the Y chromosome is quite small and contains only about 50 genes (Figure 1). This striking difference in size and gene content between the sex chromosomes makes it hard to believe that they are actually ancient partners in a pair of chromosomes that originally were very similar. Once sex became determined by a genetic signal from the Y, the sex chromosomes largely stopped recombining in germ cells. Degeneration of Y genes ensued, together with accumulation of genes that are advantageous to males on the Y chromosome, such as genes involved in testicular function and in male fertility. Similar genes appear to have accumulated on the X chromosome, so that the X chromosome also plays an important role in sperm production. The X chromosome may also have a prominent role in brain function and intelligence. A strong argument in favor of this intriguing but still controversial theory is that mental disability is more common in males.

The X Chromosome and Diseases

Some diseases affect males but not females in a family. Such diseases, called X-linked recessives, are often caused by mutations in genes located on the X chromosome, called X-linked genes. An X-linked disease is transmitted from the mother, not from the father, to an affected male, and an affected male will transmit a copy of the mutant gene to all his daughters. A famous example of an X-linked disease is hemophilia A. The blood of hemophiliac males fails to coagulate properly, leading to thinning of the blood and unstoppable bleeding after injury. This disease was recognized in the royal family of Queen Victoria, where examination of the huge pedigree readily confirmed recessive X-linked inheritance. Only males were affected, having inherited an X chromosome with a copy of a mutated gene from their healthy mothers, who were carriers of the disease. The mutated gene in hemophilia A was identified as factor VIII, a gene that encodes a protein essential for proper clotting of the blood. Males with a mutated gene cannot compensate since they have only one X chromosome, whereas female carriers have one normal gene that can compensate for the diseased gene. This typical recessive X-linked inheritance has been described for a variety of genes.

Figure 2. Karyotype of a normal human male. Each of the first twenty-two chromosomes has two copies. The last set is the sex chromosomes, and consists of one X and one Y chromosome.

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