Jason, please note that the reference you've cited actually refers to mitotic division involving division of the centromere and separation of the chromatids.
During the first meiotic division, when chiasmata form, homologous pairs of chromosomes (e.g. the two chromosome number 21's - one from the father, 1 from the mother) assemble at the cell equator. Under normal circumstances they only pair up with their corresponding partner. This can go awry when the individual carries a Robertsonian translocation. This occurs when the ends of 2 chromosomes fuse together. This causes non-disjunction and leads to extra copies (trisomy) of the affected chromosomes in the gametes and hence can cause syndromes like Down's.
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