X-Chromosome and Abnormalities

Abstract

This research investigates the function of the X chromosome and its different abnormalities on humans, including data from comparative genome analysis of other organisms. The X chromosome has many fatures that are unique in the human genome. Females inherit an X chromosome. Gene expression on one of the female X chromosomes is silenced early in development by the process of X-chromosome inactivation (XCI), and this chromosome remains inactive in somatic tissues thereafter. In the female germ line, the inactive chromosome is reactivated and undergoes meitotic recombination with the second X chromosome. The male X chromosome fails to recombine along virtually its entire length during meiosis: instead, recombination is restricted to short regions at the tips of the X chromosome arms that recombine with equivalent segments on the Y chromosome. Genes inside these regions are shared between the sex chromosomes, and their behaviour is therefore described as ‘pseudoautosomal’. Genes outside these regions of the X chromosome are strictly X-linked, and the vast majority are present in a single copy in the male genome. The unique properties of the X chromosome are a consequence of the evolution of sex chromosomes in mammals. The sex chromosomes have evolved from a pair of autosomes within the last 300 million years. In the process, the original, functional element have been conserved on the X chromosome, but the Y chromosome has lost almost all traces of the ancestral autosome, including the genes that were once shared with the X chromosome. The hemizygosity of males for almost all X chromosome genes exposes recessive phenotypes, yhus accounting for the large number of diseases that have been associated with the X chromosome.The biological consequences of the sex chromosome evolution account fort the intense interest in the human X chromosome in recent decades. However, evolutionary processes are likely to have shaped the behaviour and structure of the X chromosome in many the ways, influencing features such as repeat content, mutation rate, gene content and haplotype structure. The availability of the finished sequence of the human X chromosome, described here, now allows us to explore its evolution and unique properties at a new level.

Keywords

• X chromosome,XCI,Y chromosom

References

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