Genes and Chromosomes: The Building Blocks of Life

Every human being has 20,000 to 25,000 genes that determine the growth, development and functions of our physical and biochemical systems. Genes are normally packaged into 46 chromosomes (23 pairs) inside our cells.

The pairs numbered 1 to 22 are the same in males and females and are called autosomes. The 23rd pair are sex-determining chromosomes. Females have two Xs and males have one X and one Y.

Sperm and egg cells are different from other body cells. These reproductive cells each have only 23 unpaired chromosomes. When a single sperm and egg come together—when pregnancy begins—they form their own new cell with 46 chromosomes. The human being that results is genetically unique, with a blueprint half from each parent.