Meiosis and Genetic Variation

Meiosis is the type of cell division that produces gametes. One diploid cell divides twice to give four haploid cells, and these cells are genetically different from each other and from the parent cell. Meiosis creates this variation in two ways.Crossing over happens in prophase I. Homologous chromosomes pair up, and non-sister chromatids wrap around each other. At points called chiasmata the chromatids break and rejoin, swapping sections of DNA. Each chromatid now carries a new combination of alleles, so alleles that were on the same chromosome can be separated.Independent assortment happens in metaphase I. The homologous pairs line up on the equator in random orientation, so which chromosome of each pair (maternal or paternal) goes to each pole is independent of every other pair. The number of possible combinations is:number of combinations = 2ⁿwhere n is the number of homologous pairs. Humans have 23 pairs, so independent assortment alone gives 2²³, about 8.4 million, different combinations of chromosomes in the gametes.Independent assortment of chromatids in metaphase II adds further variation, because after crossing over the two chromatids of each chromosome are no longer identical. Together these processes mean that every gamete a person makes is almost certainly unique.