polymorphism The existence of two or more distinctly different forms (morphs) within a plant or animal species. An example is the caste system of social insects, in which there are workers, drones, and queens. This is an environmental polymorphism (see polyphenism), i.e. the differences are caused by environmental rather than genetic factors, in this case by the larvae receiving different types of food. There are also heritable or genetic polymorphisms, in which differences are the result of inherited characteristics. Two forms of genetic polymorphism exist: transient polymorphism, in which a particular form is in the process of spreading through a population, causing the relative proportion of phenotypes to alter; and balanced polymorphism, in which two or more forms coexist in a stable ratio within a population, each form possessing both advantageous and disadvantageous characteristics. An example of balanced polymorphism is the occurrence of sickle-cell disease, a genetic disease that principally affects Black populations of central Africa and is characterized by an abnormal form of the blood pigment haemoglobin (haemoglobin S) and sickle-shaped red blood cells. Three different types of individual occur in such populations: those who have two genes (AA) for normal haemoglobin and therefore do not suffer from the disease; those with one normal and one abnormal gene (AS), who are described as having the sickle-cell trait and generally suffer no symptoms; and those with two abnormal genes (SS), who suffer a chronic and eventually fatal form of anaemia. Normally such a harmful gene would have been eliminated from the population by the process of natural selection, but it is maintained in this case because people with the sickle-cell trait are resistant to a severe form of malaria endemic in central Africa. Compare mutation.

See also restriction fragment length polymorphism; single nucleotide polymorphism.

Polymorph

A polymorph is a chemical composition that can crystallize into more than one type of structure. This results in different minerals with identical compositions and distinguished by their crystallography.

Some common examples of polymorphs are calcite and aragonite. The composition of these two minerals is CaCO₃, but calcite is rhombohedral while aragonite is orthorhombic. Diamond and graphite , both of which are pure carbon , are also polymorphs. Diamond, however, is cubic while graphite is hexagonal. Pyrite is the cubic form of FeS₂, marcasite, the orthorhombic version.

A single chemical composition that can form polymorphs does so as a response to varying conditions of formation. The temperature , pressure, and chemical environment all affect the crystallization process and can determine the resulting polymorph. For example, diamond requires very high pressure to crystallize, while graphite forms at lower pressures. For the composition CaCO₃, calcite is the high temperature-low pressure polymorph while aragonite forms at higher pressures and lower temperatures.

Many polymorphs are only stable within a certain range of conditions and solid-state transitions from one polymorph to another are possible. When low-quartz, which is rhombohedral, is heated to above 1063°F (573°C), it instantaneously goes through an internal structural displacement, or shift, to form hexagonal high-quartz. This type of polymorphic transition is reversible if the temperature is lowered. Other polymorphic transitions involve extensive internal rearrangement and reconstruction of the crystal and subsequently require significantly more energy. The examples of diamond-graphite, pyrite-marcasite, and calcite-aragonite are all known as reconstructive transitions. The large amounts of energy required to effect these polymorphic changes makes the resulting mineral more stable and the process less reversible than with a displacive transition.

See also Crystals and crystallography; Mineralogy

polymorphism
1. In genetics, the existence of two or more forms that are genetically distinct from one another but contained within the same interbreeding population. The polymorphism may be transient or it may persist over many generations, when it is said to be balanced. Classical examples of polymorphisms are the presence or absence of banding in Cepaea snails, the number of spots on the wings of ladybirds, and eye colour in humans. All these are visible polymorphisms that can readily be seen in nature. Some polymorphisms, however, are cryptic and require biochemical techniques to identify phenotypic differences. Such techniques include gel electrophoresis of enzymes and other proteins, and the fragmentation of the DNA molecule by restriction enzymes (which allows the sequencing of nucleotides), both of which operate nearer to the level of the genotype.

2. In social insects, the presence of different castes within the same sex. See POLYTYPISM.

polymorphism Ability of elements or compounds to exist in more than one crystal form, with each having the same chemical composition but different physical properties due to differences in the arrangement of atoms. Examples are graphite and diamond (both C); alpha and beta quartz (both SiO₂); and calcite (hexagonal) and aragonite (orthorhombic), both forms of CaCO₃. The terms ‘dimorphism’ and ‘trimorphism’ describe (respectively) two and three different forms.

polymorphism In genetics, the existence of 2 or more forms that are genetically distinct from one another but contained within the same interbreeding population. The polymorphism may be transient or it may persist over many generations, when it is said to be balanced. Some visible polymorphism can readily be seen in nature. Other examples are cryptic and require biochemical techniques to identify phenotypic differences. Such techniques include gel electrophoresis of enzymes and other proteins, and the fragmentation of the DNA molecule by restriction enzymes (which allows the sequencing of nucleotides), both of which operate nearer to the level of the genotype. See also POLYTOPISM.