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From the Scott Freeman textbook Biological Science 4th Edition: https://www.amazon.com/Biological-Science-4th-Scott-Freeman/dp/0321598202,

Multiple allelism is defined as "the existence of more than two alleles of the same gene within a population."

Polymorphism is defined as "the occurrence of more than two distinct phenotypes of a trait in a population."

I don't really understand the difference - unless it means that two alleles may not result in two different phenotypes?

Additionally, another definition of polymorphism also in the textbook was "the existence of more than one allele at a certain genetic locus."

What does this mean? I was under the impression that all genes had at least two alleles.

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  • $\begingroup$ can you cite where you are getting your definitions? $\endgroup$
    – Vance L Albaugh
    Apr 6, 2016 at 1:22
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    $\begingroup$ There are many different meanings of polymorphism and not all involve different phenotypes. Please edit your question and give us more context. Neither of the definitions you use are really correct. $\endgroup$
    – terdon
    Apr 6, 2016 at 8:42
  • $\begingroup$ Well, my original definitions came from my teacher, but the definitions provided in my textbook have both cleared up some confusion AND yet further confused me. Which definition of polymorphism is most common? $\endgroup$
    – the real deal
    Apr 8, 2016 at 2:36
  • $\begingroup$ Can you please indicate what is your textbook and indicate on what page you found these definitions? $\endgroup$
    – Remi.b
    Apr 8, 2016 at 14:58
  • $\begingroup$ The name of the textbook was already given above, but for further detail it is the 4th edition (2011). The definitions were part of the glossary, on pages G:19 and G:23. $\endgroup$
    – the real deal
    Apr 10, 2016 at 15:59

4 Answers 4

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The term polymorphism is broad and can have different meanings. Here are your definitions

Multiple Allelism: The existence of more than two alleles of the same gene within a population.

Polymorphism: the occurrence of more than two distinct phenotypes of a trait in a population.

Considering your definitions only, then multiple allelism has to do with genetics while polymorphism has to do with the phenotype.

Most loci (=position in the genome) that contain more than one allele (=genetic variant) have no effect on the phenotype. So multiple allelism does not necessarily yield to phenotypic polymorphism.

On the other hand, plenty of phenotypic variation in a population is due to environmental variation (including what an individual eats) and not to genetic variation. So phenotypic polymorphism is not necessarily caused by a case of multiple allelism.

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This answer refers to polymorphism within genetics (for clarification), which as far as I know is the more common use of the term (but that could just be because I'm a geneticist).

Polymorphism in general refers to genetic variation of traits within a population (or species), due to mutations.

A 'single nucleotide polymorphism' is a single change in the DNA sequence of an organism at some point. So:

A -> C

or

T -> G

Or some other combination of these. It is simply a change of a single nucleotide. More specifically, a SNP is present in at least 1% of a given population.

As these mutations (and other types of mutations) accumulate it results in multiple allele variants of a gene.

Here is a good reference: http://www.nature.com/scitable/search-scitable?criteria=Polymorphism

You are right to say that not all alleles will yield a different phenotype.

Finally, in principle, genes do not necessarily have to have more than one allele, it is the result of mutations that cause this.

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  • $\begingroup$ Please add references to your answer. $\endgroup$
    – another 'Homo sapien'
    Dec 20, 2016 at 7:52
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    $\begingroup$ My apologies. Reference has just been added. $\endgroup$
    – DryLabRebel
    Dec 22, 2016 at 3:48
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Is a slightly difference:

  • polymorphism is a variation in an allele at the sequence level. A common one is Single Mutation Polymorphisms SNPs.

  • Multiple allelism means that in a population there are more than one alleles for a certain gene.

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  • $\begingroup$ Please add references to your answer. $\endgroup$
    – another 'Homo sapien'
    Dec 20, 2016 at 7:51
  • $\begingroup$ My genetic Professor $\endgroup$
    – flavinsky
    Dec 20, 2016 at 8:39
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    $\begingroup$ Welcome to Biology.SE. Please provide non-anecdotal references for your claims, i.e. information that can be checked by others as well. Also take a look at the guidelines on how to provide good answer and consider taking the tour. Have fun on Biology.SE! $\endgroup$
    – AlexDeLarge
    Dec 20, 2016 at 11:44
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unless it means that two alleles may not result in two different phenotypes?

That is correct. Due to codon degeneracy.. ie one amino acid can be encoded by more than one set of 3 DNA base pairs... you can a difference in DNA but no difference in the protein. This synonymous base changes leads to no phenotype.

Also it is possible for an amino acid change to occur in a protein and cause no phenotype either, as the change may occur in a region that has no over effect on the protein function, folding and interaction with other proteins.

So as you have eluded too, not all allelic variation leads to a phenotypic variation.

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