Figure 4

Cartoon illustrating the different types of secondary non-disjunction that may take place in a T21 oocyte. a) Formation of a bivalent plus a univalent, where the univalent is undergoing precocious disjunction leading to a chromosome 21 plus a chromatid in each of the daughter cells. b) Formation of a trivalent with chiasmata in an aberrant position, i.e. in this example distally within the long arm. Secondary non-disjunction at anaphase 1 will lead to two chromosomes 21 traveling into one of the daughter cells and one chromosome 21 into the other