Generally, aneuploidies are caused by the impaired assortment of chromosomes during meiosis. For example, they are caused when the homologous chromosomes of pair 21 do not separate and, therefore, gametes with two chromosomes 21 and gametes without chromosomes 21 are formed. If a gamete with two chromosomes 21 fertilizes a normal gamete of the opposite sex, the zygote will present trisomy (three chromosomes 21). If a gamete without chromosomes 21 fertilizes a normal gamete of the opposite sex, it will result in a zygote with monosomy (only one chromosome 21).
What is the cause of aneuploidies?