The 23 Pairs of Human Chromosomes Each person’s unique genotype is represented in the 23 pairs of chromosomes found in the nucleus of almost all human body cells. This photograph, taken through a microscope, depicts a karyotype, which shows one cell’s complete set of chromosomes. By convention, the chromosomes are arranged in pairs from largest to smallest, numbered from 1 to 22. The 23rd pair of chromosomes, called the sex chromosomes, determines a person’s biological sex. The sex chromosomes in this karyotype are XX, indicating a female. A male karyotype would have an XY combination.

CNRI/Science Photo Library/Photo Researchers