Sex chromosome aneuploidies comprise a relatively common group of chromosome disorders characterized by the loss or gain of one or more sex chromosomes. Two of the most common sex chromosome aneuplodies are Turner syndrome and Klinefelter syndrome.
Turner syndrome is a chromosomal condition that affects development in females. The condition occurs in approximately 1 in 2,500 newborn girls. The most common feature of Turner syndrome is short stature, which becomes evident by about age five. In addition, an early loss of ovarian function is also very common. Many affected girls typically do not undergo puberty and most are unable to conceive. A small percentage of females with Turner syndrome retain normal ovarian function through young adulthood. About one third to one half of individuals with Turner syndrome have heart defects which can produce complications that can be life-threatening. In addition, developmental delays, learning disabilities, and behavorial problems are possible and can vary among affected individuals.
Turner syndrome occurs when one normal X chromosome is present in the female cell instead of the normal two X chromosomes. The other X chromosome is either missing or structurally altered.
Klinefelter syndrome is a chromosomal condition that affects male physical and cognitive development. Its signs and symptoms vary among affected individuals. The condition affects approximately 1 in 500 to 1,000 newborn males. Individuals with Klinefelter syndrome typically have small testes that do not produce normal levels of testosterone. This shortage of testosterone production can lead to delayed or incomplete puberty, breast enlargement, reduced facial and body hair, and infertility.
In most cases, Klinefelter syndrome is the result of the presence of one extra copy of the X chromosome in each cell. Extra copies of genes on the X chromosome interfere with male sexual development, often preventing the testes from functioning normally and reducing the levels of testosterone. The signs and symptoms of Turner syndrome may vary among individuals afflicted with the condition.
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