 Noncrossover recombination

Noncrossover recombination is an outcome of homologous recombination without reciprocal exchange of the chromosome arms flanking the repaired region. Homologous sequence can provide a template for repair, and local sequence information can change, even though the flanking-marker arrangement remains parental. Absence of a chromosomal crossover therefore does not imply absence of recombination.^[ref]

For a schematic example, write one DNA region as $LCR$ and its homolog as $lcr$, where the outer letters are distinguishable flanking markers. Repair of the first molecule can copy a short tract containing $c$ and produce $LcR$, while retaining its original $L$–$R$ flanking combination. This is compatible with local gene conversion and a noncrossover outcome. A crossover exchanging the right-hand arms would instead alter the linkage of those distant markers.

A yeast study distinguished the outcomes experimentally by following DNA intermediates and products through meiosis. Noncrossover heteroduplex products appeared earlier than crossovers. In a mutant that accumulated unresolved Holliday junction intermediates and formed very few crossovers, noncrossover products still appeared with normal timing and abundance. The findings argued against treating all noncrossovers as merely the alternative late resolution of one common junction intermediate.^[ref]

The term classifies the final relationship of flanking sequences. It does not specify one universal repair pathway, and observing an unchanged pair of distant markers alone may miss local sequence changes.