A picture of the chromosomes in a cell is called a "karyotype," explains the National Human Genome Research Institute. There are 46 chromosomes, or 23 pairs, in a human cell. The karyotype helps identify chromosome abnormalities, which may be in the number of chromosomes or the structure of the chromosomes.
Chromosomes can be seen under the microscope if they are stained. After staining, they appear as light and dark bands. To identify the chromosomes, they are paired and numbered from one to twenty-two. The last pair are named X and Y. Males have one X and one Y chromosome, whereas females have two X chromosomes. People with Down syndrome have three copies of chromosome twenty-one instead of two.