3.10—Meiosis and genetic variation

Meiosis reduces a diploid nucleus to four haploid nuclei through two divisions. Homologous chromosomes separate in meiosis I; sister chromatids separate in meiosis II, so chromosome number halves in the first division.

Variation arises because homologous pairs align independently at metaphase I and non-sister chromatids exchange corresponding DNA at crossing-over in prophase I. Each gamete therefore receives a different combination of parental alleles.

For a species with haploid number n, independent assortment alone permits 2ⁿ chromosome combinations. In humans that is 2²³ possible combinations before crossing-over and fertilisation add further variation.

Crossing-over swaps segments between non-sister chromatids of homologous chromosomes; it is not the same as sister-chromatid separation. Meiosis produces haploid gametes, not two identical diploid cells.