A unit of the material of heredity. A gene corresponds to a particular area of DNA within a chromosome. There are about 30,000 different genes arranged on the 23 pairs of chromosomes. These genes control the development and functioning of organs and body systems, providing an “instruction manual” for an individual’s growth, survival, reproduction, and possibly also for aging and death. Genes also play a part, together with environmental factors, in determining a person’s intelligence, personality, and behaviour. Genes fulfil these functions by directing the manufacture of proteins. Many proteins have a structural or catalytic role in the body. Others switch genes “on” or “off”. The genes that make these regulatory proteins are called control genes. The activities of control genes determine the specialization of cells; within any cell some genes are active and others idle, according to its particular function. If the control genes are disrupted, cells lose their specialist abilities and multiply out of control; this is the probable mechanism by which cancers form (see carcinogenesis; oncogenes). Each of a person’s body cells contains an identical set of genes because all the cells are derived, by a process of division, from a single fertilized egg, and with each division the genes are copied to each offspring cell (see mitosis; meiosis). Occasionally, a fault occurs in the copying process, leading to a mutation. The gene at any particular location on a chromosome can exist in any of various forms, called alleles. If the effects of an allele mask those of the allele at the same location on its partner chromosome, it is called dominant. The masked allele is recessive. (See also genetic code; inheritance.)


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