Meiosis facilitates stable sexual reproduction. Without the halving of ploidy, or chromosome count, fertilization would result in zygotes that have twice the number of chromosomes as the zygotes from the previous generation. Successive generations would have an exponential increase in chromosome count.
In organisms that are normally diploid, polyploidy, the state of having three or more sets of chromosomes, results in extreme developmental abnormalities or lethality. Polyploidy is poorly tolerated in most animal species. Plants, however, regularly produce fertile, viable polyploids. Polyploidy has been implicated as an important mechanism in plant speciation.
Most importantly, recombination and independent assortment of homologous chromosomes allow for a greater diversity of genotypes in the population. This produces genetic variation in gametes that promote genetic and phenotypic variation in a population of offspring.
The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termed disjunction. When the separation is not normal, it is called nondisjunction. This results in the production of gametes which have either too many of too few of a particular chromosome, and is a common mechanism for trisomy or monosomy. Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis.
This is a cause of several medical conditions in humans (such as):
- Down Syndrome - trisomy of chromosome 21
- Patau Syndrome - trisomy of chromosome 13
- Edward Syndrome - trisomy of chromosome 18
- Klinefelter Syndrome - extra X chromosomes in males - ie XXY, XXXY, XXXXY
- Turner Syndrome - lacking of one X chromosome in females - ie XO
- Triple X syndrome - an extra X chromosome in females
- XYY Syndrome - an extra Y chromosome in males