During prophase I of meiosis, homologous chromosomes — one maternal and one paternal, each already duplicated into two sister chromatids joined at a single centromere — pair with one another in a process called synapsis, forming a four-chromatid unit called a tetrad; at points of physical overlap called chiasmata, and stabilized by the synaptonemal complex, non-sister chromatids break their DNA backbones and exchange the corresponding segments distal to that point. This exchange, termed crossing over or genetic recombination, produces chromatids carrying allele combinations found in neither parental chromosome, so that the gametes ultimately formed include recombinant types and the total number of distinct gamete genotypes rises above the two that unrecombined homologs would yield. The concept belongs to genetics and cell biology — specifically meiosis and the theory of genetic variation — and supplies, alongside independent assortment, one of the principal mechanisms by which sexual reproduction generates heritable diversity.
During prophase I of meiosis, homologous chromosomes — one maternal and one paternal, each already duplicated into two sister chromatids joined at a single centromere — pair with one another in a pro…