Accurate chromosome segregation requires homologous chromosomes in meiosis I and sister chromatids in mitosis or meiosis II to move to opposite poles. A failure of that separation is called nondisjunction.

Nondisjunction changes chromosome number without necessarily changing the internal DNA sequence of the affected chromosomes.

Nondisjunction in meiosis I

If both homologs of one chromosome pair move to the same pole during meiosis I, one daughter receives both homologs and the other receives neither.

After meiosis II, the resulting haploid products are all abnormal for that chromosome type:

meiosis-I nondisjunction
        ↓
2 products with an extra homolog
2 products missing that chromosome

Nondisjunction in meiosis II

If meiosis I is normal but sister chromatids fail to separate in one meiosis-II division, the four products have a different pattern:

2 normal products
1 product with an extra chromosome copy
1 product missing that chromosome

This difference can help identify which division failed.

Aneuploidy changes the number of individual chromosomes

A cell with an abnormal number of particular chromosomes is aneuploid.

For a normally diploid chromosome type:

• monosomy means only one copy is present instead of two;
• trisomy means three copies are present instead of two.

Aneuploidy differs from a change in ploidy. Ploidy concerns complete chromosome sets, whereas aneuploidy concerns gains or losses of individual chromosomes within those sets.

For example, a diploid cell with one extra copy of one chromosome is still based on two chromosome sets but is aneuploid for that chromosome.

Mitotic nondisjunction creates mosaic cell populations

Nondisjunction can also occur during mitosis. If one mitotic division missegregates a chromosome, the two daughter cells can acquire different chromosome numbers. Later descendants of those cells can preserve the difference, producing genetically distinct cell lineages within one organism.

Segregation errors are dosage errors

Changing chromosome copy number changes the copy number of many genes at once. The biological consequences therefore often arise from altered gene dosage, meaning altered numbers of gene copies and potentially altered amounts of their products.

Nondisjunction is consequently a chromosome-segregation error, while aneuploidy is a possible genomic state produced by that error.