Nondisjunction

Human meiosis is highly regulated, but chromosomes can sometimes fail to separate properly during cell division. This error, called nondisjunction, produces gametes with missing or extra chromosomes.

During meiosis, chromosomes or chromatids can fail to separate properly. These errors can result from incorrect homologous chromosome pairing, defects in homologous recombination, problems with cohesin proteins, or improper spindle fiber attachment.

During normal anaphase I of meiosis I, homologous chromosomes separate and move toward opposite poles. Sometimes, nondisjunction causes both homologous chromosomes to move to the same pole instead of separating.

After meiosis II is complete, nondisjunction during meiosis I produces four cells with abnormal chromosome numbers. Two cells receive an extra chromosome, while the other two cells are missing that chromosome.

Nondisjunction can also happen during anaphase II of meiosis II. Normally, at this stage, sister chromatids separate and move to opposite poles. During nondisjunction, sister chromatids may fail to separate properly.

At the end of meiosis II, four cells are produced. Two cells have normal chromosome numbers, while one cell contains an extra chromosome, and another cell is missing a chromosome.

After fertilization by a normal haploid gamete, a gamete resulting from nondisjunction can produce an aneuploid zygote. Aneuploid zygotes contain an abnormal number of chromosomes, with either extra or missing chromosomes.

In humans, this can lead to conditions such as Down syndrome, which results from an extra copy of chromosome 21, or Turner syndrome, which results from a missing X chromosome.