A karyotype is simply a picture of a person's chromosomes. In order to get this picture, the chromosomes are isolated, stained, and examined under the microscope. Most often, this is done using the chromosomes in the white blood cells. A picture of the chromosomes is taken through the microscope. Then, the picture of the chromosomes is cut up and rearranged by the chromosome’s size. The chromosomes are lined up from largest to smallest. A trained cytogeneticist can look for missing or extra pieces of chromosome.
There are 22 numbered pairs of chromosomes called autosomes>. The 23rd pair of chromosomes are the sex chromosomes. They determine an individual’s gender. Females have two X chromosomes, and males have an X and a Y chromosome.
How are the Chromosomes Numbered?
Each chromosome has been assigned a number based on its size. The largest chromosome is chromosome 1. Therefore chromosome 18 is one of the smallest chromosomes in humans.