Meiosis is the type of cell division that produces gametes, the reproductive cells of a sexually reproducing organism. During meiosis, the genetic material replicates and is divided between daughter cells so that each gamete contains a single copy of each chromosome instead of two copies.
Meiosis only occurs in primordial germ cells in the reproductive organs and not in somatic cells, which make up the majority of the cells in the body. In males, meiosis is called spermatogenesis because it produces sperm cells. In females, the process is called oogenesis because it produces egg cells, also known as ova. In spermatogenesis, four fully functioning sperm are produced at the end of meiosis. In each round of oogenesis, only one fully functioning egg is formed, and the other three cells produced are non-viable polar bodies.
In some cases, errors occur during the process of meiosis. When the chromosomes do not segregate properly during meiosis, the resulting gamete ends up with too many or too few chromosomes. In most cases, the gamete or the resulting fertilized egg does not survive. Down syndrome is an example of a genetic disorder caused by an error in meiosis. Maternal age and environmental factors may contribute to meiosis errors, although in some cases errors occur simply as a result of natural inefficiencies in the process of cell division.