X chromosome

About: X chromosome is a research topic. Over the lifetime, 9862 publications have been published within this topic receiving 407354 citations. The topic is also known as: GO:0000805 & chrX.

The origin and evolution of the pseudoautosomal regions of human sex chromosomes.

Abstract: The human X and Y chromosomes share two homologous pseudoautosomal regions (PARs) which pair and recombine at meiosis. PAR1 lies at the tips of the short arms, and the smaller PAR2 at the tips of the long arms. PAR1 contains several active genes, and has been thought to be critical for pairing and fertility. The inconsistent gene content of the PARs between different species of eutherian ('placental') mammals suggests that gene content is immaterial to function, and the failure to detect a PAR at all in some rodents and all marsupials implies that homologous pairing is not universally essential for fertility. The autosomal localization of marsupial homologues of human PAR1 genes and their co-localization with human Xp22 genes implies that the human PAR1 represents a relic of part of an autosomal region added to both X and Y chromosomes between 80 and 130 MYrBP. The same argument may be made for part of PAR2. Independent additions to the sex chromosomes of macropodid marsupials and monotremes can also be inferred from comparative mapping. We conclude that the PARs are relics of differential additions, loss, rearrangement and degradation of the Y chromosome in different mammalian lineages.

A sex-ratio Meiotic Drive System in Drosophila simulans. II: An X-linked Distorter

Abstract: The evolution of heteromorphic sex chromosomes creates a genetic condition favoring the invasion of sex-ratio meiotic drive elements, resulting in the biased transmission of one sex chromosome over the other, in violation of Mendel's first law. The molecular mechanisms of sex-ratio meiotic drive may therefore help us to understand the evolutionary forces shaping the meiotic behavior of the sex chromosomes. Here we characterize a sex-ratio distorter on the X chromosome (Dox) in Drosophila simulans by genetic and molecular means. Intriguingly, Dox has very limited coding capacity. It evolved from another X-linked gene, which also evolved de nova. Through retrotransposition, Dox also gave rise to an autosomal suppressor, not much yang (Nmy). An RNA interference mechanism seems to be involved in the suppression of the Dox distorter by the Nmy suppressor. Double mutant males of the genotype dox; nmy are normal for both sex-ratio and spermatogenesis. We postulate that recurrent bouts of sex-ratio meiotic drive and its subsequent suppression might underlie several common features observed in the heterogametic sex, including meiotic sex chromosome inactivation and achiasmy.

A general theory of sexual differentiation

Abstract: A general theory of mammalian sexual differentiation is proposed. All biological sex differences are the result of the inequality in effects of the sex chromosomes, which are the only factors that differ in XX vs. XY zygotes. This inequality leads to male-specific effects of the Y chromosome, including expression of the testis-determining gene Sry that causes differentiation of testes. Thus, Sry sets up lifelong sex differences in effects of gonadal hormones. Y genes also act outside of the gonads to cause male-specific effects. Differences in the number of X chromosomes between XX and XY cells cause sex differences in expression (1) of Xist, (2) of X genes that escape inactivation, and (3) of parentally imprinted X genes. Sex differences in phenotype are ultimately the result of multiple, independent sex-biasing factors, hormonal and sex chromosomal. These factors act in parallel and in combination to induce sex differences. They also can offset each other to reduce sex differences. Other mechanisms, operating at the level of populations, cause groups of males to differ on average from groups of females. The theory frames questions for further study, and directs attention to inherent sex-biasing factors that operate in many tissues to cause sex differences, and to cause sex-biased protection from disease. © 2016 Wiley Periodicals, Inc.

XY chromosome nondisjunction in man is associated with diminished recombination in the pseudoautosomal region.

Abstract: To assess the possible association between aberrant recombination and XY chromosome nondisjunction, we compared pseudoautosomal region recombination rates in male meiosis resulting in 47,XXY offspring with those resulting in 46,XY and 46,XX offspring. Forty-one paternally derived 47,XXYs and their parents were tested at six polymorphic loci spanning the pseudoautosomal region. We were able to detect crossing-over in only six of 39 cases informative for the telomeric DXYS14/DXYS20 locus. Subsequently, we used the data to generate a genetic linkage map of the pseudoautosomal region and found it to be significantly shorter than the normal male map of the region. From these analyses we conclude that most paternally derived 47,XXYs result from meiosis in which the X and Y chromosomes did not recombine.

Heritability of X chromosome--inactivation phenotype in a large family

Abstract: One of the two X chromosomes in each somatic cell of normal human females becomes inactivated very early in embryonic development. Although the inactivation of an X chromosome in any particular somatic cell of the embryonic lineage is thought to be a stochastic and epigenetic event, a strong genetic influence on this process has been described in the mouse. We have attempted to uncover evidence for genetic control of X-chromosome inactivation in the human by examining X chromosome-inactivation patterns in 255 females from 36 three-generation pedigrees, to determine whether this quantitative character exhibits evidence of heritability. We have found one family in which all seven daughters of one male and the mother of this male have highly skewed patterns of X-chromosome inactivation, suggesting strongly that this quantitative character is controlled by one or more X-linked genes in some families.