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<p>Examples of tightly linked genes having epistatic effects on fitness are found in supergenes and the human major histocompatibility complex genes. The effect can occur directly at the genomic level, where one gene could code for a protein preventing transcription of the other gene. Alternatively, the effect can occur at the phenotypic level. For example, the gene causing albinism would hide the gene controlling color of a person's hair. In another example, a gene coding for a widow's peak would be hidden by a gene causing baldness. Fitness epistasis (where the affected trait is fitness) is one cause of linkage disequilibrium.</p>
<p>Studying genetic interactions can reveal gene function, the nature of the mutations, functional redundancy, and protein interactions. Because protein complexes are responsible for most biological functions, genetic interactions are a powerful tool.</p>
<p><table id="toc" class="toc"> <tbody> <tr> <td> <div id="toctitle"> <h2>Contents</h2> <span class="toctoggle">[hide]</span></div> <ul> <li class="toclevel-1 tocsection-1"><span class="tocnumber">1</span> <span class="toctext">Classification by fitness or trait value</span></li> <li class="toclevel-1 tocsection-2"><span class="tocnumber">2</span> <span class="toctext">Epistasis and sex</span></li> <li class="toclevel-1 tocsection-3"><span class="tocnumber">3</span> <span class="toctext">Functional or mechanistic classification</span></li> <li class="toclevel-1 tocsection-4"><span class="tocnumber">4</span> <span class="toctext">See also</span></li> <li class="toclevel-1 tocsection-5"><span class="tocnumber">5</span> <span class="toctext">References</span></li> <li class="toclevel-1 tocsection-6"><span class="tocnumber">6</span> <span class="toctext">External links</span></li> </ul> </td> </tr> </tbody></table> </p>
<h2><span id="Classification_by_fitness_or_trait_value" class="mw-headline">Classification by fitness or trait value</span></h2>
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<div style="width: 202px" class="thumbinner"><img class="thumbimage" alt="" width="200" height="157" src="http://upload.wikimedia.org/wikipedia/commons/thumb/8/86/Synergistic_versus_antagonistic_epistasis.svg/200px-Synergistic_versus_antagonistic_epistasis.svg.png" width="200" height="157" />
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<div class="magnify"><img alt="" width="15" height="11" src="http://bits.wikimedia.org/skins-1.17/common/images/magnify-clip.png" width="15" height="11" /></div>
Diagram illustrating different relationships between numbers of mutations and fitness. <i>Synergistic</i> epistasis is the blue line - each mutation has a disproportionately large effect on the organism's fitness. <i>Antagonistic</i> epistasis is the red line. See Evolution of Sex</div>
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<div style="list-style-type: decimal" class="reflist">
<ol class="references">
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(July 2007). <a class="external text" rel="nofollow" href="http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1931480" rel="nofollow">"Coevolution of robustness, epistasis, and recombination favors asexual reproduction"</a>. <i><a class="mw-redirect" title="Proc Natl Acad Sci U S A" href="/wiki/Proc_Natl_Acad_Sci_U_S_A">Proc Natl Acad Sci U S A</a></i> <b>104</b> (31): 12801–6. <a title="Digital object identifier" href="/wiki/Digital_object_identifier">doi</a>:<a class="external text" rel="nofollow" href="http://dx.doi.org/10.1073%2Fpnas.0705455104" rel="nofollow">10.1073/pnas.0705455104</a>. <a title="PubMed Central" href="/wiki/PubMed_Central">PMC</a> <a class="external text" rel="nofollow" href="http://www.pubmedcentral.gov/articlerender.fcgi?tool=pmcentrez&artid=1931480" rel="nofollow">1931480</a>. <a class="mw-redirect" title="PubMed Identifier" href="/wiki/PubMed_Identifier">PMID</a> <a class="external text" rel="nofollow" href="http://www.ncbi.nlm.nih.gov/pubmed/17646644" rel="nofollow">17646644</a><span class="printonly">. <a class="external free" rel="nofollow" href="http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1931480" rel="nofollow">http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1931480</a></span>.</span><span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.genre=article&rft.atitle=Coevolution+of+robustness%2C+epistasis%2C+and+recombination+favors+asexual+reproduction&rft.jtitle=%5B%5BProc+Natl+Acad+Sci+U+S+A%5D%5D&rft.aulast=MacCarthy+T%2C+Bergman+A.&rft.au=MacCarthy+T%2C+Bergman+A.&rft.date=July+2007&rft.volume=104&rft.issue=31&rft.pages=12801%E2%80%936&rft_id=info:doi/10.1073%2Fpnas.0705455104&rft_id=info:pmc/1931480&rft_id=info:pmid/17646644&rft_id=http%3A%2F%2Fwww.pubmedcentral.nih.gov%2Farticlerender.fcgi%3Ftool%3Dpmcentrez%26artid%3D1931480&rfr_id=info:sid/en.wikipedia.org:Epistasis"><span style="display: none"> </span></span></li> <li id="cite_note-leclerc-5">^ <a href="#cite_ref-leclerc_5-0"><sup><i><b><font size="2">a</font></b></i></sup></a> <a href="#cite_ref-leclerc_5-1"><sup><i><b><font size="2">b</font></b></i></sup></a> <span class="citation Journal">Leclerc R. 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<h2><span id="External_links" class="mw-headline">External links</span></h2>
<ul>
<li><a class="external text" rel="nofollow" href="http://scienceaid.co.uk/biology/genetics/inheritance.html" rel="nofollow">Science Aid: Epistasis</a> High school (GCSE, Alevel) resource.</li> <li><a class="external text" rel="nofollow" href="http://www.geneticinteractions.org" rel="nofollow">GeneticInteractions.org</a></li> <li><a class="external text" rel="nofollow" href="http://www.epistasis.org" rel="nofollow">Epistasis.org</a></li>
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