Crossing over: an event where non-sister chromatidsplugin-autotooltip__default plugin-autotooltip_bigNon-sister chromatids: Chromatids that are derived from two different chromosomes in a homologous chromosome pair during meiosis I. See also: sister chromatids. exchange material with each other during meiosis Iplugin-autotooltip__default plugin-autotooltip_bigMeiosis I: the first cell division of meiosis. Crossing over occurs during Meiosis I..