Unequal Crossing Over

Abstract

Unequal crossing-over (UCO) occurs when distinct loci similar in sequence undergo homologous recombination. UCO was inferred from a series of experiments on the Bar locus in Drosophila melanogaster, demonstrating that crossing-over occurred in conjunction with Bar locus mutations. The process may result in duplication and deletion of genetic material, formation of chimeric genes, and the generation of mobile extrachromosomal elements. Contemporary studies of newly abundant genomic data show multicellular genomes to be filled with gene families whose formation can best be understood through UCO. Primates have been found to have substantial copy number variation (CNV) generated through nonallelic homologous recombination (NAHR), a type of UCO that occurs among highly similar sequences. The genetic variation produced is substantial, exceeding even single-nucleotide polymorphisms (SNPs) on a per-base-pair basis. That variation has major genetic disease-generating capability but also provides new variation for evolutionary processes. UCO plays a central role in chromosomal rearrangements (inversions, translocations), gene duplication, and tandem duplication of functional and structural modules especially in multicellular organisms, generation of gene families, and generation of mobile genetic materials such as exons – all important sources of the genetic variation necessary for evolution.