Polymorphism (redirected from balanced polymorphism)

polymorphism, of minerals, property of crystallizing in two or more distinct forms. Calcium carbonate is dimorphous (two forms), crystallizing as calcite or aragonite. Titanium dioxide is trimorphous; its three forms are brookite, anatase (or octahedrite), and rutile. Polymorphism of an element is called allotropy. The process was discovered (1821) by Eilhard Mitscherlich. See isomorphism; mineral; crystal.

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polymorphism

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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Meaning many shapes. In object technology, polymorphism is exhibited when a request (message) produces different results based on the object that it is sent to. For example, the command to show the cursor on screen displays a different icon due to its current location on screen. See object-oriented programming and polymorphic virus.

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Polymorphism (genetics)

A form of genetic variation, specifically a discontinuous variation, occurring within plant and animal species in which distinct forms exist together in the same population, even the rarest of them being too common to be maintained solely by mutation. Thus the human blood groups are examples of polymorphism, while geographical races are not; nor is the diversity of height among humans, because height is “continuous” and does not fall into distinct tall, medium, and short types. See Mutation

Distinct forms must be controlled by some switch which can produce one form or the other without intermediates such as those arising from environmental differences. This clear-cut control is provided by the recombination of the genes. Each gene may have numerous effects and, in consequence, all genes are nearly always of importance to the organism by possessing an overall advantage or disadvantage. They are very seldom of neutral survival value, as minor individual variations in appearance often are. Thus a minute extra spot on the hindwings of a tiger moth is in itself unlikely to be of importance to the survival of the insect, but the gene controlling this spot is far from negligible since it also affects fertility. See Recombination (genetics)

Genes having considerable and discontinuous effects tend to be eliminated if harmful, and each gene of this kind is therefore rare. On the other hand, those that are advantageous and retain their advantage spread through the population so that the population becomes uniform with respect to these genes. Evidently, neither of these types of genes can provide the switch mechanism necessary to maintain a polymorphism. That can be achieved only by a gene which has an advantage when rare, yet loses that advantage as it becomes commoner.

Occasionally there is an environmental need for diversity within a species, as in butterfly mimicry. Mimicry is the resemblance of different species to one another for protective purposes, chiefly to avoid predation by birds. Sexual dimorphism falls within the definition of genetic polymorphism. In any species, males and females are balanced at optimum proportions which are generally near equality. Any tendency for one sex to increase relative to the other would be opposed by selection.

In general, a gene having both advantageous and disadvantageous effects may gain some overall advantage and begin to spread because one of the features it controls becomes useful in a new environment. A balance is then struck between the advantages and disadvantages of such a gene, ensuring that a proportion of the species carry it, thus giving rise to permanent discontinuous variation, that is, to polymorphism. See Protective coloration

Polymorphism is increasingly known to be a very common situation. Its existence is apparent whenever a single gene having a distinct recognizable effect occurs in a population too frequently to be due merely to mutation. Even if recognized by some trivial effect on the phenotype, it must in addition have important other effects. About 30% of the people in western Europe cannot taste as bitter the substance phenylthiourea. This is truly an insignificant matter; indeed, no one even had the opportunity of tasting it until the twentieth century. Yet this variation is important since it is already known that it can affect disease of the thyroid gland. See Genetics, Population genetics

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Polymorphism (crystallography)

The existence of different crystal structures with the same chemical composition. If only one chemical element is present, the forms are called allotropes. Graphite and diamond are allotropes of carbon, whereas quartz and cristobalite are polymorphs of silica (silicon dioxide, SiO₂). Although properties are different in these forms, reversible transformations, which involve small shifts in atom positions and no bulk transport of material, are common. The quartz transformation at 1063°F (573°C) is a reversible, atom-displacement transformation.

In metals and ceramics, similar transformations are called martensitic. Advantage is taken of the localized nature of reversible transformation in steel by controlling the melting atmosphere, temperature, composition, mechanical working (alloying), and tempering and quenching operations.

Control over transformations to achieve desirable properties as either devices or structural materials in extreme environments is a frequent objective. In the case of tin, reversibility on the atomic scale can have devastating consequences for bulk properties. Similar transformations may be beneficial in the right place and in the desired degree. Such transformation is attempted with metals and ceramics. See Crystal structure

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(theory, programming)

polymorphism - A concept first identified by Christopher Strachey (1967) and developed by Hindley and Milner, allowing types such as list of anything. E.g. in Haskell: length :: [a] -> Int is a function which operates on a list of objects of any type, a (a is a type variable). This is known as parametric polymorphism. Polymorphic typing allows strong type checking as well as generic functions. ML in 1976 was the first language with polymorphic typing. Ad-hoc polymorphism (better described as overloading) is the ability to use the same syntax for objects of different types, e.g. "+" for addition of reals and integers or "-" for unary negation or diadic subtraction. Parametric polymorphism allows the same object code for a function to handle arguments of many types but overloading only reuses syntax and requires different code to handle different types. See also generic type variable. In object-oriented programming, the term is used to describe a variable that may refer to objects whose class is not known at compile time and which respond at run time according to the actual class of the object to which they refer.