In general what is the cause of the aneuploidies?
In general the aneuploidies are caused by impaired assortment of chromosomes during meiosis. For instance, when the homologous chromosomes of the pair 21 do not separate gametes with two chromosomes 21 and gametes without chromosomes 21 form. If the gamete with two chromosomes 21 fecundates a normal gamete of the opposite sex the zygote will present trisomy (three chromosomes 21). If the gamete without chromosomes 21 fecundates a normal gamete of the opposite sex there will be a zygote with monosomy (with only one chromosome 21).
Defects in the separation of chromosomes during cell division are called chromosomal nondisjunctions. During the meiosis nondisjunctions may occur in the anaphase I (nondisjunction of homologous) as well in anaphase II (nondisjunction of sister chromatids).