Non-disjunction

Non-disjunction is the failure of chromosomes to separate correctly during meiosis. It produces gametes with an abnormal number of chromosomes: one too many (n + 1) or one too few (n − 1).It can happen in two ways:
• In meiosis I, a pair of homologous chromosomes fails to separate in anaphase I, and both move to the same pole. Of the four gametes produced, two have an extra chromosome (n + 1) and two are missing that chromosome (n − 1), so all four are abnormal.
• In meiosis II, the sister chromatids of one chromosome fail to separate in anaphase II. Of the four gametes, one has an extra chromosome, one is missing it and two are normal.If an abnormal gamete is fertilised by a normal one, the zygote has an abnormal chromosome number. This is aneuploidy:(n + 1) + n = 2n + 1 (trisomy)(n − 1) + n = 2n − 1 (monosomy)Worked example: in humans n = 23. A sperm with 24 chromosomes fertilising a normal egg (23) gives a zygote with 47 chromosomes, a trisomy. An egg with 22 chromosomes fertilised by a normal sperm gives 45, a monosomy.Most aneuploid zygotes do not survive, because the extra or missing chromosome upsets the balance of many genes. A few survive, particularly those involving small chromosomes or the sex chromosomes; examples include Down's syndrome and Turner's syndrome.In humans the risk of non-disjunction increases with the age of the mother, possibly because egg cells stay paused in prophase I from before birth until ovulation, sometimes for over 40 years.