So, normal eggs and sperm cells only have 23 chromosomes instead of 46. This is what a normal set of chromosomes looks like. Note the 22 evenly paired chromosomes plus the sex chromosomes. The XX means that this person is a female. The test in which blood or skin samples are checked for the number and type of chromosomes is called a karyotype , and the results look like this picture.
Many errors can occur during cell division.
However, occasionally one pair doesn't divide, and the whole pair goes to one spot. This means that in the resulting cells, one will have 24 chromosomes and the other will have 22 chromosomes. If a sperm or egg with an abnormal number of chromosomes merges with a normal mate, the resulting fertilized egg will have an abnormal number of chromosomes. Hence the scientific name, trisomy 21 . The cause of the nondisjunction error isn't known, but there is definitely connection with maternal age.