0
$\begingroup$

Why is it that in biology we often say that a gene has two alleles? When we analyze allele frequencies (e.g. using Hardy-Weinberg Equilibrium), formulas are often generalized for two alleles of a gene.

Given mutations, isn't it quite likely that a population will have more than two alleles for a gene?

$\endgroup$
5
$\begingroup$

The two-allele scenario is often used in genetics teaching because of its simplicity. However, quite a few genes have more than two alleles. Some examples that readily come to mind are:

  • The ABO gene in humans: This determines ABO blood group, and has six common alleles. Many more rare alleles have been described [1].

  • The human leukocyte antigen (HLA) genes: These are well-known for their allelic diversity. One of them, HLA-A, alone has over 6,000 alleles [2].


Reference

  1. Seltsam A, Hallensleben M, Kollmann A, Blasczyk R. The nature of diversity and diversification at the ABO locus. Blood 2003; 102(8):3035–3042. https://doi.org/10.1182/blood-2003-03-0955

  2. Robinson J, Halliwell JA, Hayhurst JD, et al. The IPD and IMGT/HLA database: allele variant databases. Nucleic Acids Research 2015; 43:D423-431. http://hla.alleles.org/nomenclature/stats.html

| improve this answer | |
$\endgroup$
1
$\begingroup$

Depending at which level you are defining an allele. As @Adhish described, at the molecular level there is no theoretical limit on the number of alleles of a given gene. The presence or not of more than 2 alleles depends on evolutionary factors such as population size, fitness change, genetic drift, etc. Hardy-Weinberg Equilibrium with two alleles is easier to understand but is possible to extend the genotype predictions for multiple alleles in the equilibrium. Also remember that the Hardy-Weinberg Equilibrium is theoretical and requires assumptions that are never really met in nature, so is extremely hard for multi allele systems to ever reach the equilibrium.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.