A gene is a is a fundamental unit of heredity that consists of a sequence of DNA (deoxyribonucleic acid) occupying a section along a chromosome. Each gene contains instructions for the synthesis of a specific protein or RNA, which ultimately determines the characteristics and functions of the organism in which it exists.
The number of genes in an organism, as well as the number of genes per chromosome and the number of chromosomes, can vary greatly according to the species. For example, humans contain an estimated 20,000 to 25,000 genes, whereas bacteria typically have with 1,000 to 4,000 genes. However, the number of genes in an organism does not necessarily coordinate with its size or complexity, but rather is largely a result of evolutionary factors. In humans, the genes are distributed among 23 chromosomes, of which there are two sets, one inherited from each parent.
A mutation is a change in the structure of a gene and a process that is fundamental to biodiversity and evolution. Mutations can result from errors in DNA replication during cell division, from exposure to mutagens (radiation or some chemicals) and from viral infections. A mutation acquired during an organism's lifetime can be passed to its offspring if occurs in germ-line cells (egg and sperm cells).
Most mutations are benign and have no noticeable effect on an organism, its offspring and its environment. Others, however, are harmful and can negatively affect an organism's quality of life, its longevity, its offspring, and/or other organisms in its habitat. Still others, although very rarely, are beneficial and will increase the possibility that an organism will thrive and pass on its mutated genes, and possible new characteristics, to subsequent generations.