Where does polymorphism collide?

In genetics, polymorphism is the presence of two or more distinct forms, or alleles, of a gene or trait within one interbreeding population, at frequencies too high to be explained by recurrent mutation alone. Ford's 1945 Polymorphism defined the term and separated a balanced polymorphism, held in place by selection that favours the heterozygote, from a transient one that is on its way to becoming fixed. Allison's 1954 study of the sickle-cell allele gave the classic balanced case: the allele persists because carriers resist malaria, so two forms are kept in the population by opposing pressures. At the molecular scale the same idea names a single-nucleotide polymorphism, or SNP, a site where the DNA sequence varies between individuals, as Brookes's 1999 review set out. The referent is variation among the members of a population, counted as allele frequencies, not a crystal form, a program's types, or a body plan switched on by the environment.[1][2][3]

Unit

Two or more alleles held in one population, by their frequencies

Recovery means

A balanced polymorphism is maintained across generations when selection favours the heterozygote or varies over space and time

Measured by

Counted as allele and genotype frequencies by genotyping and sequencing, then tested against what neutral drift and mutation alone would predict

In chemistry and materials science, polymorphism is the ability of one solid compound to crystallise in more than one distinct arrangement of the same atoms or molecules, giving crystal forms that differ in structure while sharing a chemical formula. Graphite and diamond are the textbook illustration: both are pure carbon, yet their different lattices give sharply different hardness, colour, and conductivity. Haleblian and McCrone's 1969 Pharmaceutical Applications of Polymorphism showed why this matters for medicines, because different polymorphs of the same drug can dissolve at different rates and so change how much of it reaches the bloodstream. Datta and Grant's 2004 review surveyed how such forms are found, predicted, and controlled during drug development. The referent is a crystal packing of a fixed substance, identified by diffraction, and it has nothing to do with alleles in a population or interfaces in a program.[4][5]

Unit

Two or more crystal structures of one compound, at a fixed formula

Recovery means

A metastable polymorph can convert to a more stable form over time, so manufacturers control the conditions that fix which form crystallises

Measured by

Identified and told apart by X-ray diffraction, thermal analysis, and spectroscopy, and linked to dissolution and stability behaviour

In programming, polymorphism is the property that lets a single name or interface operate on values of more than one type, so that one piece of code works across many kinds of data. Cardelli and Wegner's 1985 On Understanding Types, Data Abstraction, and Polymorphism gave the standard map of its varieties: parametric polymorphism, where a function is written once and applies uniformly to any type, as with generics; inclusion or subtype polymorphism, where a value of a subtype is accepted wherever its supertype is expected, as in object inheritance; and ad-hoc polymorphism, such as overloading, where one name selects different code for different argument types. The common point is one interface standing in for many underlying types. The referent is a discipline of typed programs, checked by a compiler or dispatched at run time, and it does not name a variant in DNA, a crystal structure, or an insect caste.[6]

Unit

One interface resolved to many underlying types

Recovery means

Not applicable

Measured by

Established by a type system and checked at compile time, or resolved by dispatch at run time

In developmental biology, polymorphism, and especially the special case called polyphenism, names the capacity of a single genotype to develop into two or more discrete, alternative body forms, with the form an individual takes set by its environment rather than by which alleles it carries. The queen and worker castes of a bee colony, or the seasonal wing patterns of some butterflies, share their genes but develop down separate switch-controlled paths. West-Eberhard's 1989 review placed such developmental switches at the centre of how new forms arise, since a plastic response can later become fixed. This is a different meaning from the genetic one: there the forms track different alleles spread across a population, while here one genome yields many forms in response to cues. The referent is an organism's developmental outcome, and it shares only the word with crystal forms and program types.[7]

Unit

Two or more discrete forms produced from one genome

Recovery means

Not applicable

Measured by

Assessed by rearing one genotype under different conditions and scoring the discrete forms that result