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According to the entry for allele in Wikipedia:

“An allele is a variant form of a given gene, meaning it is one of two or more versions of a known mutation at the same place on a chromosome.”

However on Nature Scitable the definition of allele it states:

“Humans are called diploid organisms because they have two alleles one from each parent.”

These definitions disagree: one states alleles arise due to mutations on a chromosome, whereas the other states that they are the forms of a gene on a homologous pair of chromosomes.

So what is the correct definition of an allele?

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    $\begingroup$ I have edited your question, especially to include links to the sources of your quotations. In future questions be sure to do this yourself. Also please complete the Tour and read the Help on asking questions. $\endgroup$ – David Dec 15 '19 at 19:51
  • $\begingroup$ The Wikipedia definition is poorly worded, but I don't see a contradiction — alleles are specific instances of genes. This is analogous to isotopes in nuclear chemistry — every atom exists as a specific isotope. I'm voting to close because this seems to me like a very basic question that could be answered by looking at any decent undergraduate textbook or on a site like Khan Academy. $\endgroup$ – tyersome Dec 15 '19 at 20:36
  • $\begingroup$ @tyersome — Correction. You are not voting to close, because you have not yet accrued the privilege (3000) to do so. You are suggesting that the question should be closed. $\endgroup$ – David Dec 15 '19 at 22:16
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The question takes one sentence in the Nature Scitable article about alleles out of context. The statement:

“Humans are called diploid organisms because they have two alleles one from each parent.”

is not the actual definition of allele in the article, which is in the first sentence:

“An allele is a variant form of a gene.”

Rather, it is part of an explanation of the situation for variants in humans. (It is not meant to be a definition of diploid, either.)

This is perhaps clearer in the definition for allele given in the respected undergraduate text, Alberts’ Molecular Biology of the Cell, where the same two ideas follow one another directly.

Allele
One of a set of alternative forms of a gene. In a diploid cell each gene will have two alleles, each occupying the same position (locus) on homologous chromosomes.

The first sentence is the definition, the second sentence provides an example of the variants in diploid cells.

As the Wikipedia entry explains, the word allele is derived…

…from the Greek prefix ἀλληλο-, allelo-, meaning "mutual", "reciprocal", or "each other", which itself is related to the Greek adjective ἄλλος, allos (cognate with Latin alius), meaning "other".

Thus it encompasses the idea of variation or difference (rather than gene homology) which is what it was coined to describe.

FOOTNOTE

It is possible to refer to alleles that are identical, but only in a context where variation is possible. Consider the four possibilities for the mutation, m, of a wild type gene, wt.

1. wt wt
2. m  wt
3. wt m
4. m  m

We can refer to 2 and 3 as having one mutant and one wild-type allele, and 4 as having two mutant alleles (or both alleles mutant). But what about 1? It is natural to refer to this as saying both alleles are wild type, and indeed almost impossible to avoid referring to them as alleles. They are both normal, and therefore not strictly variants, but they are alternatives to the mutation at this same position.

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  • $\begingroup$ I'm clearly missing something, but I don't see the problem. My understanding is that an allele is a specific instance of a gene. $\endgroup$ – tyersome Dec 15 '19 at 20:47
  • $\begingroup$ Apologies for missing the quotation out of context in the question. I have recast my answer extensively to address this. The person that upvoted the original answer is welcome to retract his vote. $\endgroup$ – David Dec 16 '19 at 17:27

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