When meiosis goes wrong
Occasionally meiosis goes wrong. For example, Down syndrome is usually the result of an error at meiosis when an egg or sperm cell gets an extra chromosome 21. So most people with Down syndrome have one too many chromosomes. Errors at meiosis occur more often in the egg cell as women get older, so the chances of a couple having a baby with Down syndrome rises from about one in 1400 in women under the age of 25, to one in a 100 by age 40.
