A unit of heredity composed of DNA. In classical genetics (see mendelism; mendel’s laws) a gene is visualized as a discrete particle, forming part of a chromosome, that determines a particular characteristic. It can exist in different forms called alleles, which determine which aspect of the characteristic is shown (e.g. tallness or shortness for the characteristic of height).

A gene occupies a specific position (locus) on a chromosome. In view of the discoveries of molecular genetics, it may be defined as the sequence of nucleotides of DNA (or RNA) concerned with a specific function, such as the synthesis of a single polypeptide chain or of an RNA molecule, corresponding to a particular sequence of the genetic code. One or more of these structural genes, coding for protein, may be associated with other genes controlling their expression (see operon). Only about 1.5% of human genes encode proteins, although about 75% of the genome is transcribed, at least on some occasions. Much of this gives rise to various types of non-protein-coding RNA, now thought to play crucial roles in controlling the expression of structural genes. See also cistron.