Molecular unit of inherited material in a living organism. Genes contain all the information that enables an organism to build and maintain its cells and to pass genetic traits to its offspring. The entirety of the genetic information of a given organism is referred to as its genome. The inheritance of genes and the way they work is studied in genetics. Since the late 1990s, this has been complemented by the large-scale study of entire genomes, known as genomics. Genes can be manipulated using the techniques of genetic engineering.
In all organisms except viruses (which use RNA), genes are made of DNA located at a fixed point (locus) on linear chromosomes. A gene consistently affects a particular character (phenotype) in an individual and may have several variants (alleles), each specifying a particular form of that character. For example, the gene for eye colour has alleles for blue or brown eyes. Some alleles show dominance and mask the effect of other alleles, known as recessive. This is called Mendelian inheritance, after Austrian biologist Gregor Mendel, who first proposed this mechanism for inherited genetic results as a result of his study of plant breeding.
One gene carries the information that describes how one particular polypeptide (or a stable RNA molecule, such as a transfer RNA) 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 which the organism lives.
In higher organisms, 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 (see human genome), 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 known as a mutation. Mutations can cause an individual to have a disease or disorder, such as Huntington's disease, cystic fibrosis, or sickle-cell anaemia.
The term ‘gene’ was coined in 1909 by the Danish Geneticist Wilhelm Johannsen.
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.
Effect of chromosomes and genes on health
Chromosomes and genes
DNA: Discovery of the Structure of DNA
Process of genetic engineering
The Human Genome Project
Genes and selective breeding
Gene manipulation in cloning and genetic engineering
Inheritance and selective breeding
I Can Do That
Question of Genes: Inherited Risks
Your Genes, Your Choices: Exploring the Issues Raised by Genetic Research
inside the human body
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