Editing Isozyme (section)

==Introduction==
Isozymes were first described by [[R. L. Hunter]] and [[Clement Markert]] (1957) who defined them as ''different variants of the same enzyme having identical functions and present in the same individual''.<ref>{{Cite journal|last1=Markert|first1=Clement L.|last2=Moller|first2=Freddy|year=1959|title=Multiple forms of enzymes: tissue, ontogenetic, and species specific patterns.|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=45|issue=5|pages=753–763|doi=10.1073/pnas.45.5.753|pmc=222630|pmid=16590440|doi-access=free}}</ref>  This definition encompasses (1) enzyme variants that are the product of different genes and thus represent different [[locus (genetics)|loci]] (described as ''isozymes'') and (2) enzymes that are the product of different [[alleles]] of the same gene (described as ''allozymes'').<ref name="Kearney">{{cite book |last1=Kearney |title=Fundamental Genetics |date=2014 |publisher=McNaughton Publishing |pages=413–414 |edition=3rd}}</ref>

Isozymes are usually the result of [[gene duplication]], but can also arise from [[polyploidy|polyploidisation]] or [[nucleic acid hybridization]]. Over evolutionary time, if the function of the new variant remains ''identical'' to the original, then it is likely that one or the other will be lost as [[mutation]]s accumulate, resulting in a [[pseudogene]]. However, if the mutations do not immediately prevent the enzyme from functioning, but instead modify either its function, or its pattern of [[gene expression|expression]], then the two variants may both be favoured by [[natural selection]] and become specialised to different functions.<ref>{{cite book |last1=Gerald, Gerald |title=The Biology Book: From the Origin of Life to Epigenetics, 250 Milestones in the History of Biology |date=2015 |publisher=Sterling |page=79}}</ref> For example, they may be expressed at different stages of development or in different tissues.<ref name="Huang">{{cite book |last1=Huang, Le |title=Genome |date=2009 |publisher=Grady-McPherson |page=299}}</ref>

Allozymes may result from [[point mutation]]s or from insertion-deletion ([[mutation|indel]]) events that affect the coding sequence of the gene. As with any other new mutations, there are three things that may happen to a new allozyme:

* It is most likely that the new allele will be non-functional—in which case it will probably result in low [[fitness (biology)|fitness]] and be removed from the population by [[natural selection]].<ref name="Alberts">{{cite book |last1=Alberts |title=Molecular Biology of the Cell |date=2017 |publisher=Garland Science |page=649 |edition=6th}}</ref>
* Alternatively, if the [[amino acid]] residue that is changed is in a relatively unimportant part of the enzyme (e.g., a long way from the [[active site]]), then the mutation may be [[Neutral theory of molecular evolution|selectively neutral]] and subject to [[genetic drift]].<ref name="Walstrom">{{cite journal |last1=Walstrom, Ford|display-authors=etal |title=Models of genetics and natural selection: a current biomolecular understanding |journal=Biomolecular Ecology |date=2014 |volume=70 |issue=2 |pages=1021–1034}}</ref>
* In rare cases, the mutation may result in an enzyme that is more efficient, or one that can catalyse a slightly different [[chemical reaction]], in which case the mutation may cause an increase in fitness, and be favoured by natural selection.<ref name="Walstrom" />