Exchange of DNA during meiosis between improperly aligned segments of DNA that can result in a gain or loss of DNA. Circumstances that predispose to unequal crossing over are misalignment of: (1) highly homologous segment duplications (low copy repeats) referred to as nonallelic homologous recombination which result in recurrent deletions or duplications; and (2) a gene and its pseudogene in tandem on a chromosome (e.g., CYP21 and its pseudogene CYP21P; GBA1 [formerly GBA] and its pseudogene GBA1LP [formerly GBAP]) which result in de novo pathogenic variants.

See nonallelic homologous recombination.

Related terms: deletion; duplication; gene conversion; recombination