Where does allele collide?

One of the alternative heritable forms that a gene can take at a given position on a chromosome, transmitted from parent to offspring as a discrete unit and often showing dominance or recessivity over its partner form. The idea traces to Gregor Mendel's 1866 breeding experiments on peas, and the word is a shortening of the term allelomorph that William Bateson and Edith Saunders introduced in 1902 for such paired alternative characters. Here an allele is a physical variant an organism inherits and can express as a visible trait, not a proportion tracked across a population, an entry in a sequence catalogue, or a value stored at a position in a computer program.[1][2]

Unit

One inherited variant form of a gene at one locus

Recovery means

Not applicable

Measured by

Read from breeding crosses and phenotype ratios, or from genotyping that distinguishes the alternative forms

In the study of how populations change across generations, an allele is one of the variant types present in a shared gene pool, and the quantity that matters is its frequency, the proportion of all copies at that locus that are of this type. G. H. Hardy's 1908 note in Science showed that, absent disturbing forces, these proportions stay fixed from one generation to the next, which made the changing allele frequency the basic bookkeeping variable of evolution under selection, drift, mutation, and migration, later formalised by Sewall Wright in 1931. Here the allele is an abstract type defined by its share of the pool rather than a single inherited copy, a catalogued sequence, or a slot in a program.[3][4]

Unit

The proportion of copies at a locus that are of one type

Recovery means

Not applicable

Measured by

Estimated by counting variant types in population samples and modelled as frequency change over generations

In the typing of the human leukocyte antigen genes for transplantation matching and disease studies, an allele is a variant defined by its exact DNA sequence and given a formal name by an international committee, so that a single gene can carry tens of thousands of distinct alleles that may differ by as little as one base. Marsh and colleagues' 2010 report in Tissue Antigens set out the naming rules by which each newly found sequence becomes a numbered allele in the reference database. Here the allele is a precisely sequenced, registered entity identified by its nucleotides, unlike a dominant or recessive form seen in a cross, a proportion in a population, or a value in a chromosome of code.[5]

Unit

One DNA-sequence variant of a gene, named in the reference registry

Recovery means

Not applicable

Measured by

Determined by sequencing the gene and matching the result to a named entry in the HLA database