Molecular mechanism of unequal crossing over

A common mechanism of interstitial deletions is unequal crossing over in meiosis I. When highly homologous repeats ("Repeat") flank a DNA segment ("Gene"), two non-sister chromatids may recombine at an incorrect location due to inappropriate alignment. Thus, one of the chromatids (labeled "A") acquires a duplication of the intervening sequence, while the corresponding segment is deleted on the other chromatid and only one repeat remains (labeled "B").