Gametes are reproductive cells, such as sperm and eggs. They contain only one copy of each gene, instead of the two copies that are found in other cells in a mature pea plant. Because of this, gametes can combine to form offspring with new combinations of alleles.

This reduction in the number of alleles per gene is called meiosis. Meiosis is a specialized form of cell division that occurs only in reproductive cells. During meiosis, the chromosomes in a cell are replicated (copied) and then the resulting daughter cells are divided into four, each with half the number of chromosomes as the original cell.