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The X chromosome in humans spans more than 153 million [[base pairs]] (the building material of [[DNA]]). It represents about 800 protein-coding genes compared to the Y chromosome containing about 70 genes, out of 20,000–25,000 total genes in the human genome.
Each person usually has one pair of sex chromosomes in each cell. Females typically have two X chromosomes, whereas males typically have one X and one [[Y chromosome]]. Both males and females retain one of their mother's X chromosomes, and females retain their second X chromosome from their father. Since the father retains his X chromosome from his mother, a human female has one X chromosome from her paternal grandmother (father's side), and one X chromosome from her mother. This inheritance pattern [[#Inheritance pattern|follows the Fibonacci numbers]] at a given ancestral depth.{{cn}}
[[Genetic disorder]]s that are due to [[mutation]]s in genes on the X chromosome are described as [[X-linked recessive inheritance|X linked]]. If the X chromosome has a genetic disease gene, it always causes illness in male patients, since men have only one X chromosome and therefore only one copy of each gene. Females, instead, may stay healthy and only be carrier of genetic illness, since they have another X chromosome and possibility to have healthy gene copy. For example, [[hemophilia A]] and [[hemophilia B|B]] and [[congenital red–green color blindness]] run in families this way.
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