Down’s syndrome results from people having an extra copy of chromosome 21 in their cells as shown here. people with down’s syndrome have both a lower iq and a shorter life expectancy than those without down’s syndrome. this is an example of

The answer is trisomy 21.

Trisomy 21 is a condition in which an extra copy of chromosome 21 is present. Instead of two chromosomes 21 in a cell, there are three chromosomes. This is a consequence of a failure of chromosome 21 to separate during the gamete development. Thus, an egg cell or a sperm cell will have two instead of one chromosome 21 and after a cross with normal gamete cell, a zygote will have three chromosomes 21.