Center: Primary oocyte containing diploid set of chromosomes with the doubled amount of chromatin (4n) prior to maturation. Upper Panel: Normal segregation of homologues in the first meiotic division, resulting in the extrusion of the first polar body (PB1) (smaller circle) containing one of the homologues. Accordingly, the resulting secondary (metaphase II) oocyte contains the remaining homologue with two chromatids. Left Panel: Chromosome nondisjunction leading to the extrusion of PB1 containing both homologues, which results in nullisomy of this chromosome in MII oocyte (top left). Similarly the extruded PB1 may not contain any material, so both homologues will be left in MII oocyte, resulting in disomic oocyte (bottom left). Right Panel: Chromatid malsegregation, leading to either a single chromatid extrusion with PB1 (top right), which results in the extra chromatid material in MII oocyte, or to extra chomatid extrusion with PB1 resulting in the lack of one chromatid in MII oocyte (bottom right). Bottom: Cromatid or chromosome malsegregation involving different chromosomes, resulting in different types of errors of different chromatids or chromosomes in MII oocyte.
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