Nondisjunction

Meiosis normally splits chromosome number in half, so each gamete gets exactly one copy of every chromosome. Nondisjunction breaks that rule at one specific step: a pair fails to separate and both copies travel to the same pole instead of splitting up. Where this happens changes the pattern. In anaphase I, the homologous pair sticks together and moves as one unit into a single cell. That cell then goes through meiosis II normally, splitting its sister chromatids apart, but since it started with two copies instead of one, both meiosis II products inherit an extra chromosome. The other cell from meiosis I had zero copies of that chromosome to begin with, so both of its products end up missing it entirely. Result: four gametes, two with n+1, two with n-1, none normal.

Nondisjunction in meiosis II looks different because by that point the homologs already separated correctly in meiosis I. The error is sister chromatids failing to split apart. Only one of the two meiosis I cells is affected, so only two of the four final gametes are abnormal, one n+1 and one n-1, while the other two gametes are completely normal haploid cells.

When an n+1 or n-1 gamete fuses with a normal n gamete at fertilization, the zygote ends up with 2n+1 or 2n-1 chromosomes, a condition called aneuploidy. Depending on which chromosome is involved, this can be lethal, or it can produce a viable but altered phenotype, as with trisomy 21 in humans. This topic sets up how chromosome number connects to inheritance patterns and chi-square analysis, since aneuploidy shows up as unexpected ratios or phenotypes that don't fit a simple Mendelian cross.