Most organisms have two copies of each of their chromosomes. If something happens to one chromosome in a cell, the cell can sometimes use the correct information on the other chromosome. Even so, whether you're a fly or a human, incorrect information can be fatal.

Scientists study the number, size, shape, and banding patterns (dark marks) of an individual's chromosomes when they make a karyotype. A karyotype is an organized picture of an individual's chromosomes. By examining karyotypes, scientists can identify some chromosomal problems that could lead to a disease.