Eastern Illinois University The Keep Faculty Research & Creative Activity Biological Sciences January 1999 Epistasis Henry R. Owen Eastern Illinois University, [email protected] Follow this and additional works at: https://thekeep.eiu.edu/bio_fac Part of the Biology Commons Recommended Citation Owen, Henry R., "Epistasis" (1999). Faculty Research & Creative Activity. 146. https://thekeep.eiu.edu/bio_fac/146 This Article is brought to you for free and open access by the Biological Sciences at The Keep. It has been accepted for inclusion in Faculty Research & Creative Activity by an authorized administrator of The Keep. For more information, please contact [email protected]. Epistasis Field of study. Oassical transmission gest ways in which the action of genes can genetics control a phenotype and thus supply a more Significance: EpiStasis is the interaction of genes complete understanding of the influence of and their effect an the final performance or exter- genotype on phenotype. nal appearance (phenotype) of an organism. Elu- A gene can influence the expression of other cidation of the interactions between genes leads to genes in many different ways. One result of a more complete understa·nding ofthe influence of multiple genes is that more phenotypic classes genotype on phenotype, and thus it is important can result than can be explained by the action far the improuement of quantitatively inherited of a single pair of alleles. The initial evidence characteristics, such as crop yie/4, and for gaining for this phenomenon came out of the work of a greater knowltdge of the mechanisms of com- Bateson and British geneticist Reginald C. Pun- plexly inherited diseases, such as cancer. nett during their investigations on the inheri- tance of comb shape in domesticated chickens. Key terms The leghorn breed has a "single" comb, brah- ALLELE: an alternate form of a gene mas have "pea" combs, and wyandottes have OIHYBRID CRoss: a cross between parents that "rose" combs. Crosses between brahmas and differ by two specified genes wyandottes have "walnut" combs. Intercrosses Ft: first filial generation, or the progeny result- among walnut types show four different types ing from the first cross in a series of F2 progeny, in the ratio 9 walnut: 3 rose: 3 F2: second filial generation, or the progeny pea: 1 single. This ratio of phenotypes is consis- resulting from cross of the F 1 generation tent with the classical F2 ratio for dihybrid in- Introduction heritance. The corresponding ratio of geno- The term "epistasis" is of Greek and Latin types, therefore, would be 9 A_ B_: 3 A_ bb: 3 aa origin, m eaning "to 3tand upon" or "stoppage." B_: 1 aa bb, respectively. (The c;ymbol M_"is used The term was originally used by geneticist Wil- to indicate that the second gene can be either liam Bateson at the beginning of the twentieth dominant or recessive; for example, A_ means century to define genes that mask the expres- that both AA and Aa wiU result in the same sion of other genes. The gene at the initial phenotype.) In this example, one can recog- location (locus) ts termed the epistatic gene. nize that two independently assorting genes The genes at the other loci are "hypostatic" to can affect a single trait. If two gene pairs are the initial gene. In its strictest sense, it describes acting epistatically, however, the expected a nonreciprocal interaction between two or 9:3:3:1 ratio of phenotypes is altered in some more genes, such that one gene modifies, sup- fashion. Five basic examples of tw~ene epi- presses, or otherwise influences the expression static interactions can be described: comple- of another gene affecting the same phenotypic mentary, modifying, inhibiting, masking, and (physical) character or process. By this defini- duplicate gene action. tion, simple additive effects of genes affecting a single phenotypic character or process would Two-Gene Epistatic Interactions not be considered an epistatic interaction. For complementary gene action, a domi- Similarly, interactions between alternative nant allele of two genes is required to produce forms (alleles) ofa single gene are governed by a single effect. An example of this form of dominance effects and are not epistatic. Epi- epistasis again comes from the observations of static effects are interlocus interactions. There- Bateson and Pun nett of flower color in crosses fore. in terms of tht> roral genetic contribution between two white-flowered varieties of sweet to phenotype, three factors are involved: domi- peas. In their investigation, crosses between nance effects, additive effects, and epistatic ef- these cwo varieties produced an unexpected fects. The analysis of epistatic effects can sug- result: All of the F1 progeny had purple flowers. William Bateson (at right), who pioneered the study ofepi stasis, in a 1924 pose with another genetics pionee~; Wilhelm Johannsen. (California Institme of Technology) EPISTASIS 181 A Punne tt Square Sh4ctwing Flower Pigme ntation White X White CCpp ! ccPP Purple Ft CcPp ! CP Cp cP cp CCPP CCPp CcPP CcPp CP purple purple purple purple CCPp CCpp CcPp Ccpp Cp purple white purple white CcPP CC:Pp ccPP ccPp cP purple purple white white CcPp Ccpp ccPp ccpp cp purple white white white When white-jlowertd SW«t pw plants were crossed, thefirst -generation progrny (FJ) aU had purple flowers. When these plants were ~lffertiliud, the ~cond-gtmration progeny (F2) reuealed a ratio ofnine purple to seven white. This result can be explained by the presence of two gtne.s for j1qwer pigmmJation, P (dominant) or p (recessive) and Cor c. Both dominant forms, P and C, must be presenl in order to produce purple flowers. When the F 1 individuals were allowed to self- would have white fl owers. In summary, the phe- fertilize and produce the F2 generation, a phe- notypic ratio of the F2 generation would be 9:7. notypic ratio of nine purple-flowered to seven The term "modifying gene action" is used to white-flowered individuals resulted. Their hy- describe a situation whereby one gene pro- pothesis for this ratio was that a homozygouts duces an effect only in the presence of a domi- recessive genotype for either gene (or both.) nant allele of a second gene at another locus. resulted in the lack of flower pigmentation. A An example of this type of epistasis is aleurone simple model to explain the biochemical basiis color in corn. The aleurone is the outer cell for this type of flower pigmentation is a two-step layer of the endosperm (food storage tissue) of process, each step controlled by a separate the grain. In this system, a dominant gene (P_) gene and each gene having a recessive allele produces a purple aleurone layer only in the that eliminates pigment formation. Given th:is presence of a gene for red aleurone (R_) but explanation, each parent must have had com- expresses no effect in the absence of the second plemen tary genotypes (AA bb and aa BB) , and gene in its dominant form. Thus, the corre- thus both were white-flowered. Crosses be- sponding F2 phenotypic ratio is 9 purple: 3 red: tween these two parents would produce double 4 colorless. The individuals without aleurone heterozygotes (Aa Bb) with purple flowers. In pigmentation would, therefore, be of the geno- 9 1 the F2 generation, 116 would have the genotype type P_ rr (h6) or pp rr ( 116). Again, a two-step A_ B_ and would have purple flowers. The biochemical pathway for pigmentation can be remaining 7116 would be homozygous recessive used to explain this ratio; however, in this ex- for at least one of the two genes and, therefore, ample, the product of the second gene ( R) acts 182 EPISTASIS first in the biochemical pathway and allows for phenotype of the individual. It may be argued the production of red pigmentation and any that duplicate gene action is not a form of further modifications to that pigmentation. epistasis, since there may be no interaction Thus, the phenotypic ratio of the F2 generation between genes (if the two genes code for the would be 9:3:4. same protein product), but this situation may Inhibiting action occurs when one gene acts be an example of gene interaction when two as an inhibitor of the expression of another genes code for similar protein products in- gene. In this example, the first gene allows the volved in the same biochemical pathway and phenotypic expression of a gene, while the their combined interaction determines the fi- other gene inhibits it. Using a previous exam- nal phenotype of the individual. An example of ple (the gene Rfor red aleurone color in corn this type of epistasis is illustrated by seed cap- seeds), the dominant form of the first gene R sule shape in the herb shepherd's purse. In this does not produce its effect in the presence of example, either gene in its dominant form will the dominant form of the inhibitor gene /. In contribute to the final phenotype of the indi- other words, the genotype R_ i_ results in a vidual (triangular shape). If both genes are in phenotype of red aleurone (3/16), while all their recessive form, the seed capsule has an other ?enotypes result in the colorless pheno- ovoid shape. Thus, the phenotypic ratio of the type ( 3/16). Thus gene R is inhibited in its F2 generation is 15:1. expression by the expression of gene /. The F2 phenotypic ratio would be 13:3. This ratio, un- Impact and Applications like the previous two examples, includes only Nonallelic gene interactions have consider- two phenotypic classes and highlights a compli- able influence on the overall functioning of an cating factor in determining whether one or individual.