According to Mendel's Law of Segregation, each body cell has two genes for every trait. One gene is donated by the mother, and the other by the father. However, when the organism makes sex cells, egg or sperm, the gene pairs separate and only one is donated to the sex cell. The scientific name given to sex cells is gametes. The process in which sex cells or gametes are produced is called meiosis.

Let us use Mendel's pea plants once again as an example. A purebred (homozygous) tall plant has two tall genes in every cell. The genes separate during meiosis and only one tall gene is donated to the sperm or egg cells. (We will consider sperm cells.)

Parent Plant
Body Cells
Sex Cells - Gametes

Parried Tall