The human body is made up of cells. For example, when you have a sunburn your skin peels, and you are shedding skin "cells." In the center of each cell is an area called the nucleus. Human chromosomes are located in the nucleus of the cell. A chromosome is a structure in the nucleus that contains your genes. Your genes determine your traits, such as eye color and blood type. The usual number of chromosomes in each cell of your body is 46 total chromosomes, or 23 pairs. You inherit half of your chromosomes (one member of each pair) from your biological mother, and the other half (the matching member of each pair) from your biological father. Scientists have numbered the chromosome pairs from 1 to 22, with the 23rd pair labeled as Xs or Ys, depending on the structure. The first 22 pairs of chromosomes are called "autosomes." The 23rd pair of chromosomes are known as the "sex chromosomes," because they determine whether someone will be born male or female. Females have two "X" chromosomes, and males have one "X" and one "Y" chromosome. A picture of all 46 chromosomes, in their pairs, is called a karyotype. A normal female karyotype is written 46, XX, and a normal male karyotype is written 46, XY.