In the normal process of meiosis, chromosomes pair at the metaphase plate and subsequently separate and migrate to opposite poles. When nondisjunction occurs, the chromosomes do not properly separate. Instead, both members of one homologous chromosome pair migrate to the same pole. As a result, half the gametes will have an extra chromosome, and half will be missing a chromosome. Gametes with missing or with extra chromosomes are usually sterile. However, sometimes, certain chromosome imbalances are fertile. These almost always lead to genetic defects. Down syndrome occurs when an egg or sperm with an extra number 21 chromosome fuses with a normal gamete. The result is a zygote with three copies of chromosome 21 (trisomy 21). Down syndrome individuals bear various abnormalities, including mental retardation, heart defects, respiratory problems, and deformities in external features. Turner syndrome results when there is nondisjunction of the sex chromosomes. Sperm will have either both chromosomes (XY) or no chromosome (O, used to indicate the absence of a chromosome). Similarly, eggs will be either XX or O. A Turner syndrome zygote (XO) is a female who has one X (from a normal egg or sperm) and no second chromosome (from nondisjunction in an egg or sperm). Individuals with Turner syndrome are physically abnormal and sterile.
