gene

Basic unit of inherited material, encoded by a strand of DNA and transcribed into RNA. In higher organisms, genes are located on the chromosomes. A gene consistently affects a particular character in an individual – for example, the gene for eye colour. Also termed a Mendelian gene, after Austrian biologist Gregor Mendel, it occurs at a particular point, or locus, on a particular chromosome and may have several variants, or alleles, each specifying a particular form of that character – for example, the alleles for blue or brown eyes. Some alleles show dominance. These mask the effect of other alleles, known as recessive. Genes can be manipulated using the techniques of genetic engineering (gene technology). The entirety of the genetic information of a given organism is referred to as its genome.

The inheritance of genes and the way genes work is studied in genetics. Since the late 1990s, this has been complemented by the large-scale study of entire genomes, known as genomics.

One gene carries the information that describes how one particular protein is made. This information is stored as a chemical code on a DNA molecule, together with regulatory elements. Each protein that is made helps to determine part of the characteristics of an organism. Between them, all the proteins determine all the inherited characteristics of an organism, though some of these characteristics can be modified by the environment. In higher organisms, the DNA is located in the chromosomes in the nucleus of a cell. Thousands of genes are present on each chromosome. The total number of genes in a human, according to estimates published in 2004 by the Human Genome Project, is thought to be between 20,000 and 25,000, distributed between the 23 pairs of chromosomes in each human cell. Occasionally, a gene or a larger part of a chromosome or the number of chromosomes becomes accidentally altered. Such a change is a mutation. Mutations can cause an individual to have a disease or disorder, such as Huntington's disease, cystic fibrosis, or sickle-cell anaemia. Gregor Mendel was the first to understand the mechanism of inheritance by genes, as a result of the study of plant breeding. He did not, however, know about the existence of DNA.

History of genetics In the 1940s, it was established that a gene could be identified with a particular length of DNA, which coded for a complete protein molecule, leading to the ‘one gene, one enzyme’ principle. Later it was realized that proteins can be made up of several polypeptide chains, each with a separate gene, so this principle was modified to ‘one gene, one polypeptide’. However, the fundamental idea remains the same, that genes produce their visible effects simply by coding for proteins; they control the structure of those proteins via the genetic code, as well as the amounts produced and the timing of production.

In modern genetics, the gene is identified either with the cistron (a set of codons that determines a complete polypeptide) or with the unit of selection (a Mendelian gene that determines a particular character in the organism on which natural selection can act). Genes undergo mutation and recombination to produce the variation on which natural selection operates.

The term ‘gene’ was coined in 1909 by the Danish Geneticist Wilhelm Johannsen (1857–1927).