Polymorphism

A variation in the DNA that is too common to be due merely to new mutation. A polymorphism must have a frequency of at least 1% in the population. Examples of polymorphisms include the genes for sickle cell disease, thalassemia and G6PD deficiency. These genes are widely believed to offer an advantage against malaria.

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Occurrence in more than one form; existence in the same species or other natural group of more than one morphologic type. SYN: pleomorphism.

- balanced p. a unilocal trait in which two alleles are maintained at stable frequencies because the heterozygote is more fit than either of the homozygotes. SEE ALSO: overdominance.

- corneal endothelial p. a greater than normal variation in the shape of the cells.

- DNA p. a condition in which one of two different but normal nucleotide sequences can exist at a particular site in DNA.

- genetic p. the occurrence in the same population of multiple discrete alletic states of which at least two have high frequency (conventionally of 1% or more).

- lipoprotein p. heritable variations in low-density β-lipoproteins; the variant lipoproteins exhibit different antigenic and chemical properties when compared with normal lipoproteins.

- restriction fragment length p. (RFLP) used in genetic analysis of populations or individual relationships. In regions of the human genome not coding for proteins there is often wide sequence variety between individuals that can be measured; in effect, the distance (in nucleotides on the chromosome) can be different, usually because of repeated base patterns.

- restriction length p., fragment length p. the existence of allelic forms recognizable by the length of fragments that result when the nucleotide chain is treated by a specific restriction enzyme that cleaves wherever a particular sequence of nucleotides occurs. A mutation in this sequence changes cleaving and hence the number of fragments.

- restriction-site p. DNA p. in which the sequence of one form of the p. contains a recognition site for a particular endonuclease, but the sequence of the other form lacks such a site.

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poly·mor·phism .päl-i-'mȯr-.fiz-əm n the quality or state of existing in or assuming different forms: as

a ) (1) existence of a species in several forms independent of the variations of sex (2) existence of a gene in several allelic forms (3) existence of a molecule (as an enzyme) in several forms in a single species

b) the property of crystallizing in two or more forms with distinct structure

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n.

(in genetics) a condition in which a chromosome or a genetic character occurs in more than one form, resulting in the coexistence of more than one morphological type in the same population.

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poly·mor·phism (pol″e-morґfiz-əm) [poly- + morph- + -ism] 1. the existence within a population or species of several different forms of individuals, or the occurrence of different forms or stages in an individual over time. 1. genetic p.