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The X chromosome is best known as one of the two chromosomes that determine sex in many animals, including humans. Most people are born with at least one X chromosome, since it is genetically inherited from the mother. Genetic mutations in X chromosomes are linked to many developmental disorders, while irregular numbers or functions of the chromosome are also connected to some medical syndromes.
When an egg is released, it already contains X chromosomes from the mother. Sperm contains a corresponding chromosome that can be either X, resulting in a female, or Y, resulting in a male. Almost all biological females have an XX chromosome pair, while almost all biological males have an XY makeup.
During early fetal development in females, the body busily shuts down one of every two X chromosomes in all cells except those used to create eggs. This process, known as lyonization, ensures that only one active copy remains to handle most cellular function. Interestingly, the shut-down of the chromosome appears to be largely random, so the paternal chromosome may be lyonized in some cells, while the maternal chromosome is silenced in others. Males do not undergo this process, as they usually only have one active X chromosome.
Chromosomes are made up of genes; the X chromosome is believed to have somewhere between 1000-2000 genes, many of which have nothing to do with determining sex. This is far greater than the amount of genes believed to inhabit Y chromosomes, which may have less than 100 genes. Genetically, this makes a certain amount of sense, as almost all mammals have X chromosomes, but generally only males have Y chromosomes.
X chromosome-linked disorders tend to occur for one of two reasons: genetic mutation or unusual chromosome combinations. Genetic mutation occurs when one or more of the genes behaves or develops abnormally and can result in many medical conditions, including increased risks for some cancers and hypophosphatemic rickets. Developmental issues, such as Kinefelter syndrome, Turner syndrome, and Triple X syndrome, are often related to extra copies of X chromosomes, such as XXY or XXXY combinations.
Research on the function and disorders of X chromosomes is far from complete. The large amount of genes and astounding amounts of possible variations continue to make the exploration of this building block component an active area of study and investigation. By gaining better understanding of the chromosome and its functions, scientists hope to develop treatments for a wide variety of X-linked disorders, as well as improve understanding of reproductive science.