A gene is a small piece of the genome. As an atom is the fundamental unit of matter, a gene is the fundamental unit of heredity. Different genes determine the different characteristics, or traits, of an organism. The number of genes in the genome varies from species to species. More complex organisms tend to have more genes.
Bacteria have several hundred to several thousand genes.
Estimates of the number of human genes, by contrast, range from 25,000 to 30,000. Heredity is an old idea. Since ancient times, people have known that offspring tend to resemble their parents. Physical appearance, temperament, and many other traits are passed down from generation to generation. But the concept of the gene dates back only to the late 19th century.