Meiosis

The process of meiosis does not work perfectly every time, and mistakes in the formation of gametes are a major cause of genetic disease in humans. Under normal conditions, the four chromatids of a tetrad will separate completely, with one chromatid going into each of the four daughter cells. In a disorder known as nondis-junction, chromatids do not separate and one of the resulting gametes receives an extra copy of the same chromosome. The most common example of this mistake in meiosis is the genetic defect known as Down syndrome, in which a person receives an extra copy of chromosome 21 from one of the parents. Another fairly common form of nondisjunction occurs when the sex chromosomes (XX, XY) do not divide properly, resulting in individuals with Klinefelter syndrome or Turner syndrome. Other mistakes that can occur during meiosis include translocation, in which part of one chromosome becomes attached to another, and deletion, in which part of one chromosome is lost entirely. The severity of the effects of these disorders depends entirely on the size of the chromosome fragment involved and the genetic information contained in it. Modern technology can detect these genetic abnormalities early in the development of the fetus, but at present, little can be done to correct or even treat the diseases resulting from them.