X chromosome
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Larger of the two sex chromosomes, the smaller being the Y chromosome. These two chromosomes are involved in sex determination. In humans, whether a person is male or female is determined by the particular combination of the two sex chromosomes in the body cells. 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). The Y chromosome is shorter than the X. Genes on these chromosomes determine a person's sex. Genes carried on the X chromosome produce the phenomenon of sex linkage.
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 becomes condensed so that most of its genes are inactivated. If this inactivation is incomplete, skeletal defects and mental retardation result.
| 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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