X chromosome

The 23 pairs of chromosomes of a normal human male.

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.