X Chromosome
The X chromosome is one of two sex chromosomes and is present in nearly all animals that undergo sexual reproduction. Depending on the organism, it may occur alone, in two copies, or paired with a Y chromosome. The sex of an individual is directly related to the presence or absence of specific genes located on these chromosomes.
In humans, at least one X chromosome is required for life to exist. It is usually paired with either another X in a female or a Y in a male. The X chromosome is much larger than the Y, and, with the exception of a small region at one end of the X and Y chromosomes, the pseudoautosomal region, the genes present on the two chromosomes are very different. Genes located uniquely on the X chromosome are said to be X-linked.
Females have two X chromosomes and, thus, two copies of each X-linked gene. In order to express an X-linked recessive disease, a female must have two mutant genes. If only a single mutation is present, the female will not be affected with the disorder but is a mutation carrier and has a 50:50 chance of transmitting that mutation on to her offspring. Female children will also be carriers unless they receive a second mutation from the father. Males, however, have only a single X chromosome, so if a male child inherits an X-linked mutation from his mother, he will be affected with the disease. X-linked dominant disorders are expressed in both males and females, but are often lethal in males.
An interesting feature of the X chromosomes in females is that one of the pair is always turned off, or inactivated, in every cell. This mechanism equalizes the amount of protein produced from X-linked genes between males and females.
The fact that X-linked recessive traits are clearly expressed in males, allowed early geneticists to easily map genes to the X chromosome. There are currently 445 known genes on the X chromosome that are involved in a many different functions. The gene for colorblindness is found on the distal long arm of the X chromosome. Flanking this locus are a pair of genes, Factors VIII and IX, that are important in blood clotting and are associated with hemophilias A and B. Mutations in the gene retinitis pigmentosum cause a devastating disease of the eyes. Steroid sulfatase mutations produce a disorder characterized by scalyness of the skin that is known as ichthyosis. The most common inherited form of mental retardation, fragile X syndrome, is another X-linked gene.
As expected, there are also many genes involved in sex determination. Genes on the short arm are critical for female development, and loci on the long arm have been associated with female fertility. The androgen receptor gene functions in male development, and if this is nonfunctional, testicular feminization may occur. This disorder results from the interruption of the male differentiation pathway and gives rise to an individual with a female body, but no functional internal genitalia. In the 1950s and 1960s, the Soviet Bloc countries entered these women in Olympic events since they were able to out compete their rivals. It was suggested that the higher levels of testosterone resulted in increased strength. Once the situation was identified, Olympic officials decided to test all participants to ensure that the sex chromosome complement matched the physical appearance of each individual.
Imbalance in the number of sex chromosomes gives rise to a diverse set of disorders. The occurrence of three copies of an X chromosome has few adverse consequences, and individuals are apparently normal, fertile females. Apparently normal, fertile males can have one X and two Y chromosomes. However, a single X chromosome gives rise to a female individual with Turner syndrome who is short, usually infertile, with a possible learning deficit and other phenotypic anomalies. A male with two X and one Y chromosomes has Klinefelter syndrome and is usually tall, infertile, with some type of learning deficit and development of female secondary sex characteristics.
The X chromosome is critical to normal human development and has proven to be one of the most unusual of the chromosome set due to the unique feature of X inactivation.
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