Unequal crossing over

Unequal crossing over is a genetic event involving gene duplication or deletion, where one sequence is replaced by another from a sister chromatid or homologous chromosome during mitosis or meiosis. It occurs between misaligned homologous sequences and is driven by sequence similarity, which increases the likelihood of such events. This process can result in tandem repeats on one chromosome and deletions on another, with repeated sequences around duplications enhancing the frequency of unequal crossing over.

Consequences for organisms include regional gene duplications, which can lead to dosage imbalances and be deleterious due to the homogenization of duplicated sequences. Evolutionarily, unequal crossing over facilitates large sequence exchanges, such as 20,000 base pairs in yeast rDNA, compared to smaller transfers via gene conversion. It may also influence genome structure, as seen in beta-globin genes with long introns potentially evolving to mitigate harmful effects.

Duplicated genes face various evolutionary fates: becoming pseudogenes, retaining functionality for dosage effects, neofunctionalization, or subfunctionalization. Unequal crossing over is a primary driver of gene duplications, contributing significantly to genome size evolution. Additionally, it plays a role in generating repetitive DNA sequences, which can constitute large portions of eukaryotic genomes without functional constraints.