The X chromosome is one of the two sex chromosomes in humans (the other is the Y chromosome). The sex chromosomes form one of the 23 pairs of human chromosomes in each cell. The X chromosome spans about 155 million base pairs (the building blocks of DNA) and represents approximately 5 percent of the total DNA in cells.
Each person normally has one pair of sex chromosomes in each cell. Females have two X chromosomes, while males have one X and one Y chromosome. Early in embryonic development in females, one of the two X chromosomes is randomly and permanently inactivated in somatic cells (cells other than egg and sperm cells). This phenomenon is called X-inactivation or Lyonization. X-inactivation ensures that females, like males, have one functional copy of the X chromosome in each body cell. Because X-inactivation is random, in normal females the X chromosome inherited from the mother is active in some cells, and the X chromosome inherited from the father is active in other cells.
Some genes on the X chromosome escape X-inactivation. These genes are located at the tip of the short (p) arm of the X chromosome in an area known as the pseudoautosomal region. Although many genes are unique to the X or Y chromosome, genes in the pseudoautosomal region are present on both chromosomes. As a result, men and women each have two functional copies of these genes. Many genes in the pseudoautosomal region are essential for normal development.
Identifying genes on each chromosome is an active area of genetic research. Because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies. The X chromosome likely contains between 900 and 1,400 genes.
Genes on the X chromosome are among the estimated 20,000 to 25,000 total genes in the human genome.
Researchers have identified inherited genetic variants that may predict the loss of one copy of a woman's two X chromosomes as she ages, a phenomenon known as mosaic loss of chromosome X, or mLOX.
Researchers at the Francis Crick Institute, working with University of Oxford, University of York and Oxford Archaeology, have developed a new technique to measure the number of chromosomes in ancient genomes more precisely, using it to identify the first prehistoric person with mosaic Turner syndrome (characterized by one X chromosome instead of two [XX]), who lived about 2500 years ago.
The lack of some genes in the BEC/TCEAL cluster could be related to some alterations associated with the autism spectrum disorder, according to a preclinical study published in the journal Genome Biology, and led by Professor Jordi Garcia Fernàndez, from the Faculty of Biology and the Institute of Biomedicine of the University of Barcelona (IBUB), and researcher Jaime Carvajal, from the Andalusian Centre for Developmental Biology - University Pablo de Olavide (CSIC-UPO).
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