crossing overmedical dictionary

<genetics, molecular biology> The breaking during meiosis of one maternal and one paternal chromosome, the reciprocal exchange of corresponding sections of DNA along pairs of homologous chromosomes by symmetrical breakage and crosswise rejoining of the chromosomes.

This results in the transfer of a block of genes from each chromosome to its homologue. In contrast to genetic recombination, which is a phenotypic phenomenon, crossing-over is genotypic. Any even number of crossing-overs between two loci will cancel out phenotypically and no recombination will occur.

Recombination can result from DNA exchange of alleles between between homologous chromatids in meiosis, giving rise to chiasmata.

Compare: recombination.

(10 Nov 1998)