In many species, including humans, females produce eggs and males produce sperm. An egg and sperm unite beginning the development of a new individual. Sexually produced offspring are never identical to either of their parents. Each organism requires a set of instructions for specifying its traits. Heredity is the passage of these instructions from one generation to another.
Hereditary information is contained in genes, located in the chromosomes of each cell.
Each gene carries a single unit of information, and an inherited trait of an individual can be determined by either one or many genes. A human cell contains many thousands of different genes. The characteristics of an organism can be described in terms of a combination of traits. Some traits are inherited and others result from interactions with the environment. The cell is the basic unit of living things.