Change in the pool of genes of a small population that takes place strictly by chance. Genetic drift can result in genetic traits being lost from a population or becoming widespread in a population without respect to the survival or reproductive value of the gene pairs (alleles) involved. A random statistical effect, genetic drift can occur only in small, isolated populations in which the gene pool is small enough that chance events can change its makeup substantially. In larger populations, any specific allele is carried by so many individuals that it is almost certain to be transmitted by some of them unless it is biologically unfavourable.
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Discontinuous genetic variation that results in the occurrence of several different forms or types of individuals among the members of a single species. The most obvious example of polymorphism is the separation of most higher organisms into male and female sexes. Another example is the different blood types in humans. A polymorphism that persists over many generations is usually maintained because no one form has an overall advantage or disadvantage over the others in terms of natural selection. Some polymorphisms have no visible manifestations. The castes that occur in social insects are a special form of polymorphism that results from differences in nutrition rather than from genetic variation.
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Alteration in the genetic material of a cell that is transmitted to the cell's offspring. Mutations may be spontaneous or induced by outside factors (mutagens). They take place in the genes, occurring when one base is substituted for another in the sequence of bases that determines the genetic code, or when one or more bases are inserted or deleted from a gene. Many mutations are harmless, often masked by the presence of a dominant normal gene (see dominance). Some have serious consequences; for example, a particular mutation inherited from both parents results in sickle-cell anemia. Only mutations that occur in the sex cells (eggs or sperm) can be transmitted to the individual's offspring. Alterations caused by these mutations are usually harmful. In the rare instances in which a mutation produces a beneficial change, the percentage of organisms with this gene will tend to increase until the mutated gene becomes the norm in the population. In this way, beneficial mutations serve as the raw material of evolution.
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Artificial manipulation, modification, and recombination of DNA or other nucleic-acid molecules in order to modify an organism or population of organisms. The term initially meant any of a wide range of techniques for modifying or manipulating organisms through heredity and reproduction. Now the term denotes the narrower field of recombinant-DNA technology, or gene cloning, in which DNA molecules from two or more sources are combined, either within cells or in test tubes, and then inserted into host organisms in which they are able to reproduce. This technique is used to produce new genetic combinations that are of value to science, medicine, agriculture, or industry. Through recombinant-DNA techniques, bacteria have been created that are capable of synthesizing human insulin, human interferon, human growth hormone, a hepatitis-B vaccine, and other medically useful substances. Recombinant-DNA techniques, combined with the development of a technique for producing antibodies in great quantity, have made an impact on medical diagnosis and cancer research. Plants have been genetically adjusted to perform nitrogen fixation and to produce their own pesticides. Bacteria capable of biodegrading oil have been produced for use in oil-spill cleanups. Genetic engineering also introduces the fear of adverse genetic manipulations and their consequences (e.g., antibiotic-resistant bacteria or new strains of disease). Seealso biotechnology, molecular biology.
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Sequence of nucleotides in DNA and RNA that determines the amino acid sequence of proteins. A messenger RNA molecule synthesized from the DNA directs the synthesis of the protein. Three adjacent nucleotides constitute a unit known as a codon; each codon codes for a single amino acid. There are 64 possible codons, 61 of which specify the 20 amino acids that make up proteins. Because most of the 20 amino acids are coded for by more than one codon, the code is called degenerate. Once thought to be identical in all forms of life, the genetic code has been found to vary slightly in certain organisms and in the mitochondria of some eukaryotes.
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