Trisomy 21

Trisomy 21 is the name given to the non-disjunction  of homologous pairs of chromosomes 21 which leads to Down syndrome. "Trisomy" because, due to a nondisjunction event (failed separation of the chromosome pairs to separate during stage 1 or 2 of meiosis), there is an extra copy of chromosome 21 in one of the gametes. At fertilisation, this gamete will cause the zygote to contain 3 copies of the chromosome 21.