How can non-disjunction lead to down syndrome?

Non- disjunction in meiosis can take place at both anaphase I and II. When there is an error in the separation of homologous chromosomes (anaphase I) or sister chromatids (anaphase II), one of the gametes produced by the replicative error will have one chromosome too many and the other will have one chromosome too few.Down syndrome occurs when a gamete which is diploid at chromosome 21 (has a chromosomal excess) fuses with a normal haploid gamete. This produces a gamete with a trisomy at chromosome number 21 which causes down syndrome.