Recombination

Recombination is the outcome that crossing over produces: because the DNA segments exchanged at a chiasma are broken and then rejoined onto the partner chromatid, the chromatids that emerge afterward carry new combinations of alleles, called recombinants, that were not present together on either original parental chromosome. In a tetrad carrying alleles A/a at one locus and B/b at a linked locus, if a single chiasma forms between the two loci, the segments distal to that chiasma are swapped, so two of the four resulting chromatids end up as the new combinations Ab and aB (recombinant), while the other two chromatids remain unchanged as AB and ab (non-recombinant, parental type). The accepted molecular explanation for how this exchange actually happens at the level of the DNA double helix is Robin Holliday's 1964 hybrid-DNA model. In this model, two homologous DNA duplexes first align closely, side by side, alongside their already-duplicated copies; an enzyme called endonuclease then nicks one strand of each duplex at the identical position; the two cut strands cross over and anneal to the homologous strand of the partner duplex, generating a characteristic four-stranded structure called a Holliday junction; this junction can then slide along the DNA in a process called branch migration, extending a stretch of heteroduplex DNA (DNA where the two strands originated from different parental molecules …