Nondisjunction

The most common example of non-fatal trisomy in humans is that of Down syndrome, caused by the presence of an extra copy of chromosome 21. Affected individuals suffer from mental retardation, congenital heart disease, and increased suceptibility to infection. Physical characteristics include a short, stocky body, flattened facial features, and almond-shaped eyes.

Down syndrome is an example of an autosomal trisomy as it involves one of the autosomes ("autosome" is the term that designates all the chromosomes other than the X and Y, or sex, chromosomes). There are many human conditions that are caused by nondisjunction of the sex chromosomes, and these usually affect the individual's secondary sexual characteristics and fertility.

For example, the fusion of an XY sperm with a normal X egg, or the fusion of a Y sperm with an XX egg gives rise to an XXY individual (with normal autosomes). This condition is known as Klinefelter's syndrome. Individuals affected by this disorder usually have below-average intelligence. They are are phenotypically male, but present some female secondary sexual characteristics. They may develop breasts, and they have little facial hair, very small testes and are sterile.

Individuals with Turner's syndrome (XO) are females with a single X chromosome. They are sterile and have underdeveloped secondary sexual characteristics, and they are shorter than normal. Females with genetic constitution XXX, on the other hand, have a normal appearance and are fertile, but suffer from a mild mental handicap. Similarly, XYY males have relatively few clinical symptoms and appear phenotypically normal. They are taller than average and may show aggressive behavior and a below-average intelligence. Both XXX and XYY conditions usually pass undiagnosed.