Meiosis takes place only in reproductive organs generating sperm, eggs, or spores. In humans, male gametes are produced by meiosis in the testes, while female gametes are produced by meiosis in the ovaries. In human females, the primary oocyte is arrested with paired homologous chromosomes in prophase I for decades with meiosis only being completed in the monthly cycle of oogenesis which begins after puberty.
In addition to the fact that meiosis produces four daughter cells, rather than two as in mitosis, the process of crossing over also distinguishes meiosis from mitosis. The independent assortment of chromosomes in Anaphase I is a category of genetic recombination. Furthermore, the formation of the synapsis as the homologous chromosomes form the four-chromatid structure, the tetrad, leads to crossing over, another form of genetic recombination. Through crossing over and the random distribution of parental chromosomes, meiosis is fundamental to hereditary variability, a primary engine of evolution.