Genes are the physical units of heredity and are located along each chromosome in the cells of the human body. For each physical trait--eye color, height, hair color--a person inherits two genes or two groups of genes, one from each parent. Because both of these genes cannot be expressed together, one usually overpowers the other. The more powerful gene is called the dominant gene and the weaker, the recessive gene.

All genes on the same chromosome are called linked genes because they are usually inherited together. Genes on the X and Y chromosomes are called sex-linked because the X and Y chromosomes are the ones that determine sex.

Sometimes genes on the same chromosome are not inherited together. When sex cells divide to form an egg or sperm cell (a process called meiosis), each chromosome pairs off with a partner. As the chromosomes lie side by side, groups...