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have plenty of cells with the normal gene remaining functional. Sometimes, there is even cell selection in carrier females, leading to skewed X inactiva-tion in favor of the normal gene remaining functional. One intriguing feature of X inactivation is that it does not affect all X-linked genes. About 20 percent of genes "escape" X inactivation in humans. With a higher expression level in females than in males, such genes could perhaps play a role in female-specific functions. In males, some of these unusual genes have retained a functionally similar gene on the Y, as remnants of the ancient partnership of the sex chromosomes.

X inactivation was discovered in 1961 by Mary Lyon, a British scientist who studied mice. Thus, another name for this phenomenon is "Lyoniza-tion." The physiologic or normal regulation of expression of many genes is at the level of the individual gene. In contrast, X inactivation regulates a whole chromosome that comprises a huge number of genes. Special mechanisms of regulation evolved to initiate X inactivation through the action of a master gene on the X. Once one of the two X chromosomes (maternal or paternal) is randomly chosen to become inactivated in a given fetal cell, it will be faithfully maintained in this state in the progeny of the cell. The stability of the inactivation is mediated by a series of complex molecular changes epigenetic not involving called epigenetic modifications. X inactivation is lost in only one type of DNA sequence change cells, the female germ cells, where both X chromosomes are functional for transmission to the next generation. Thus, X inactivation involves special mechanisms of initiation, maintenance, and reactivation. Much work still needs to be done to fully understand the fascinating roles of the X chromosome and its regulation. see also Chromosomal Aberrations; Fragile X Syndrome; Hemophilia; Inheritance Patterns; Intelligence; Meiosis; Mosaicism; Sex Determination; Y Chromosome.

Christine M. Disteche

Bibliography

Miller, Orlando J., and Eeva Therman. Human Chromosomes. New York: SpringerVerlag, 2001.

Nussbaum, Robert L., Rod R. McInnes, and Huntington F. Willard. Thompson & Thompson Genetics in Medicine. Philadelphia, PA: Saunders, 2001.

Wang, Jeremy P., et al. "An Abundance of X-linked Genes Expressed in Spermatogonia." Nature Genetics 27, no. 4 (2001): 422-426.

Y Chromosome diploid possessing pairs of chromosomes, one member of each pair derived from each parent haploid possessing only one copy of each chromosome

The diploid human genome is packaged within 46 chromosomes, as two pairs of 23 discrete elements, into all cells other than the haploid gametic egg and sperm cells. During the reproductive process, each parent's gametes contribute 22 nonsex chromosomes and either one X or one Y chromosome.

Paternal Inheritance

The X and Y chromosomes are the sex chromosomes for mammals, including humans. Not only are the X and Y sex chromosomes in mammals physically distinctive, with the Y being smaller, the Y chromosome is exceptionally peculiar. The X chromosome contains considerably more genes than the Y, which has its functionality essentially limited to traits associated with being male. It is the Y chromosome that carries the major masculinity-determining gene (SRY, for sex-determining region Y), which dictates maleness. In a mating pair, if the paternal partner contributes a normal Y chromosome, male gonadal tissues (testes) develop in the offspring. Only males have the potential to transmit a Y chromosome to the next generation, and thus the father's contribution is decisive regarding an offspring's sex.

Since normally only one Y chromosome exists per cell, no pairing between X and Y occurs at meiosis, except at small regions. Normally, no crossing over occurs. Therefore, except for rare mutations that may occur during spermatogenesis, a son will inherit an identical copy of his father's

Y chromosome, and this copy is also essentially identical to the Y chromosomes carried by all his paternal forefathers, across the generations. This is in contrast to the rest of his chromosomal heritage, which will be a unique mosaic of contributions from multiple ancestors created by the reshuffling process of recombination.

Sex Determination and Y Chromosome Genes

While SRY is the most dramatic gene affiliated with the Y chromosome, about thirty other genes have been identified. Some notable representatives include AZFa, b, and c, which are associated with spermatogenesis and male infertility, SMCY, associated with the immune response function responsible for transplantation rejection when male tissue is grafted to female tissue, and TSPY, which may play a role in testicular cancer.

Sex Chromosome Evolution and Peculiarities

Discussions of sex chromosome evolution raise the question of the biological risks and benefits of sexual differentiation in organisms. Overall, sexual dimorphism enhances diversity that, in turn, improves the chances for evolutionary change and potential survival during periods of environmental change.

There are risks in the specialization of the Y chromosome, however. Besides its absence in females, lack of recombination for most of its physical territory except at its tips, and the strict pattern of paternal inheritance, the solitary cellular existence of the Y chromosome reduces the opportunity for DNA repair, which normally occurs while pairing during mitosis. This may explain the prevalence of multicopy DNA sequences on the Y, and why many of its genes have lost functionality. In fact, while genes predominately specific to male function tend to accumulate on the Y chromosome, other genes that have functional counterparts elsewhere will atrophy over evolutionary time, through the accumulation of uncorrected mutations. Thus the

Y chromosome is slowing evolving toward a composition with fewer and fewer essential genes.

Molecular Anthropology Using the Y Chromosome

The field of molecular anthropology is predicated on the concept that the genes of modern populations encode aspects of human history. By studying the degree of genetic molecular variation in modern organisms, one can, in principle, understand past events. The Y chromosome is uniquely suited to such studies. Secondary applications of Y chromosome variation studies

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