A diploid cell that undergoes meiosis typically produces four haploid cells, which have precisely half the genetic material of the parent cell. The diploid cell has two complete sets of chromosomes, ad each of the haploid cells has a single complete set of chromosomes.
Non disjunction refers to an error in cell division. In some cases homologous chromosomes do not separate from each other during cell division. In other cases, the problem occurs when sister chromatid do not separate from each other. Non disjunction can occur in mitosis or meiosis.
If non disjunction occurs in meiosis 1, both chromosomes of a homologous pair migrate to the same pole, leaving one daughter cell without a chromosome. The cells go through meiosis 2 normally, producing four gametes with unusual numbers of chromosomes. Two gametes have one complete set of chromosomes plus one extra chromosome, indicated by n+1, the other two gametes lack a chromosome are designated n-1.