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  A mutation, in genetics, is a change in a gene (or chromosome) which produces a change in phenotype as a result of a change in the gene product. Mutations are important in evolution because they cause variation within the gene pool. They occur at random and are thus nearly all damaging, causing the death of the individual or rendering it unable to survive in the face of competition; such genes are usually deleted from the gene pool as their carriers do not successfully reproduce. Occasionally, a mutation will occur which, by chance, renders the individual better suited to its environment; such a gene can spread through the population at an increased rate, giving rise to variation.

At a molecular level, the term mutation describes a change in sequence of nucleotides in the chromosonal DNA. A mutation can occur in any cell, but those mutations which occur during the production of gametes are of importance in terms of the population and of evolution, because they have the potential to be inherited. Some mutations (such as those that result in Down\'s syndrome) include changes in the number or structure of chromosomes. Most mutations are gene mutations and involve the addition, deletion or, most commonly, the substitution of one or more nucleotides. Since the nucleotide sequence of DNA codes for the amino acid sequence of protein product, the effect of even a single nucleotide substitution can have far-reaching effects upon the development and life of the individual, by altering the structure and function of the protein. Diseases such as haemophilia and sickle-cell anaemia are the result of relatively simple gene mutations.

Mutations can occur as the result of errors in copying during DNA replication, but this process normally has a high level of failure. External factors such as ionizing radiation (x-rays, gamma rays, etc.) and certain chemicals (pesticides, mustard gas) can greatly increase the mutation rate. RB

See also adaptation; genetic code; teratology; toxicology.Further reading Bruce Alberts, The Molecular Biology of the Cell; , Richard Dawkins, The Selfish Gene.



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