Humans possess around 30,000 genes which are the biological units of heredity. They are arranged along the length of the 23 pairs of CHROMOSOMES and, like the chromosomes, therefore come in pairs (see GENETIC CODE). Human beings have 46 chromosomes, comprising two sex chromosomes and 44 autosomes, but there is also a MITOCHONDRIAL chromosome outside the cell nucleus (see CELLS) which is inherited from the mother.

Half of a person's genes come from the father and half from the mother, and this mix determines the offspring's characteristics. (A quarter of a person's genes come from each of the four grandparents.) Genes fulfil their functions by controlling the manufacture of particular PROTEINS in the body. The power that genes have to influence the body's characteristics varies: broadly, some are dominant (more powerful); others are recessive (less powerful) whose functions are overridden by the former. Genes are also liable to change or mutate, giving the potential for the characteristics of individuals or their offspring to be altered. (See GENETIC CODE; GENETIC DISORDERS; GENE THERAPY; HUMAN GENOME.)