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Definition: x-chromosome from Philip's Encyclopedia

One of the two kinds of sex-determining chromosome; the other is the y-chromosome. In many organisms, including humans, females carry two x-chromosomes in their diploid cell nuclei, while males carry one x- and one y-chromosome. Non-sexual characteristics are also carried on the x-chromosome: for example, the genes for one form of colour blindness and for haemophilia. See also genetics; heredity


Summary Article: X chromosome
from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Larger of the two sex chromosomes in humans and other placental mammals, the smaller being the Y chromosome. In females both the sex chromosomes are the same – two X chromosomes (XX). In males the two are different – one X chromosome and one Y chromosome (XY). As the presence of a specific genes on the Y chromosome determines a person's sex, individuals with a different number of X chromosomes can still be female if they lack this factor. As the X chromosome contains many more genes than the Y chromosome, the phenomenon of sex linkage, that is the gender-specific appearance of an inherited trait, is commonly associated with genes on the X chromosome.

As a result of meiosis gametes from a female each contain one X chromosome. However, gametes from a male are of two kinds. Half of the gametes contain an X chromosome and half contain a Y chromosome. If an X-carrying gamete from a male fertilizes a female gamete the result will be a female. If a Y carrying gamete from a male fertilizes a female gamete, the result will be a male.

Early in the development of a female embryo, one of the X chromosomes in each cell becomes condensed so that most of its genes are inactivated, adjusting the gene dosage to the level corresponding to the male cell, where only one copy of the X chromosome is present. If this inactivation is incomplete, skeletal defects and mental retardation may result. A visible manifestation of X inactivation is the coat pattern of tortoiseshell cats, which results from different colour genes in paternal and maternal X chromosomes.

In rare cases, people may carry just one or more than two sex chromosomes. Having only a single sex chromosome (X) results in Turner syndrome. Individuals affected are female but with a characteristic range of physical symptoms including infertility.

In 2005 scientists at the Sanger Institute in the UK announced the determining of 99.3% of the genome of the human X chromosome. The DNA sequence showed that 10% of the genes that made up the chromosome were members of the ‘testis-antigen’ family, known to have links with cancer in male reproductive organs. This discovery could lead to the development of new treatments based on targeting testis-antigen genes.

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