Figure 3. Meiotic chromosomal missegregation resulting from pericentric crossovers.

Red and blue lines depict homologs (duplex DNA); red and blue dots depict centromeres. Crossovers, depicted by red-blue junctions, in chromosomal arms are essential for proper segregation of chromosomes during meiosis, but crossovers too near the centromere are harmful. Normally during MI, the centromeres of homologs with their attached chromosomal arms segregate to opposite poles, whereas in MII the sister centromeres segregate to opposite poles, giving rise to four haploid nuclei. During non-disjunction (NDJ) of homologs in MI, which primarily arises due to lack of crossovers in the arms, both homologs migrate to the same pole and then segregate properly at MII, giving rise to two nullisomes (nuclei lacking a chromosome) and two disomes (nuclei with two chromosome copies). Precocious separation of sister chromatids (PSSC) occurs when cohesion is lost between the sister chromatids and can occur in either MI or MII. Crossovers too close to the centromeres (pericentric crossovers) are associated with such PSSC events. In MI PSSC, sister centromeres of one homolog segregate to opposite poles at MI followed by proper MII, giving rise to a nullisome and a disome containing homologous centromeres. In MII PSSC, sisters stay in the same nucleus at MI but missegregate at MII, giving rise to a nullisome and a disome containing sister centromeres. MII NDJ has proper MI but aberrant segregation at MII, resulting in a fate similar to MII PSSC. These aberrant events are also linked to the presence of pericentric crossovers. Some of these missegregation events give rise to similar types of aneuploids, but careful tetrad analysis with multiple markers can distinguish them.