Dictionary:
e·pis·ta·sis (ĭ-pĭs'tə-sĭs)  |
[Greek, stoppage, from epistanai, to stop, check : ep-, epi-, epi- + histanai, to place.]
epistatic ep'i·stat'ic (ĕp'ĭ-stăt'ĭk) adj.| 5min Related Video: epistasis |
| Genetics Encyclopedia: Epistasis |
Epistasis, first defined by the English geneticist William Bateson in 1907, is the masking of the expression of a gene at one position in a chromosome, or locus, at one or more genes at other positions. Epistasis should not be confused with dominance, which refers to the interaction of genes at the same locus. The human genome contains from 30,000 to 70,000 gene loci. Some of them are involved in numerous interactions, making it difficult to identify their role in development and metabolism. As we learn more about the human and other genomes, it becomes clear that the borrowed phrase "no gene is an island" is an appropriate expression to describe the interplay among gene loci.
Puzzling Inheritance Patterns Explained
There are many examples of epistasis. One of the first to be described in humans is the Bombay phenotype, involving the ABO blood group system. Individuals with this phenotype lack a protein called the H antigen (geno-type hh), which is used to form A and B antigens. Even though such individuals may have A or B genes, they appear to be blood group O because they lack the H antigen.
Another well-known example is coat color in mice. Two coat-color loci are involved. At locus A, color is dominant over albino (lack of pigment). At locus B, the coat color agouti is dominant over black. A mouse that is homozygous for the albino gene will show no pigment regardless of its genotype at the other locus. Thus the A and B loci are epistatic.
It is likely that the phenomenon of lack of penetrance, in which a dominant gene fails to be expressed, is often due to epistasis. There are many cases where dominant disorders, such as polydactyly (in which individuals have extra fingers or toes), appear to "skip generations." The nonexpression of the dominant gene is likely due to the alleles the individual has at an independent locus that is epistatic to the polydactyly locus. Lack of penetrance may also be accompanied by variable expressivity, where a gene is only partially expressed. As the molecular basis of these disorders becomes known, the reason for nonpenetrance will be easier to determine.
Such interactions between loci probably occur in the genetic etiology of complex traits such as the psychiatric disorders schizophrenia and manic depression. David Lykken, a genetic psychologist at the University of Minnesota, coined the term "emergenesis" to describe multiple gene interactions involved in a specific complex trait. After comparing EEG (electroencephalogram, or "brain wave") data from identical and fraternal twins, Lykken concluded that multiple-level interactions of independent or partly independent genes must be involved.
Epistatic interactions make it difficult to identify loci conferring risk for complex disorders, and they may be a major reason that researchers have made only slow progress in mapping susceptibility genes for complex disorders. To locate interacting loci involved in the genetic origins of complex diseases requires collecting DNA samples from a large number of families where two or more individuals have the disorder. Such large-scale studies are usually difficult to conduct.
Interactions Among Proteins
As the Bombay phenotype demonstrates, it is actually proteins, not the genes, that interact. After identifying interacting loci, the next step is determining the proteins that the genes at those loci encode, and the properties of those proteins.
The emerging field that involves the study of proteins and protein interactions is called proteomics. New techniques are now available to locate proteins that interact with one another. In the yeast two-hybrid system, one such technique, one protein is used as bait, and a pool of unknown proteins, referred to as prey proteins, are tested to see if any of them bind to the bait. Binding, if it occurs, triggers a reaction that causes yeast cells to turn blue. In one experiment testing a protein's interactions with more than 1,000 other proteins, 950 interactions were found. Not all of these interactions are likely to occur or be important in the organism, but such results indicate how common, and complex, protein interactions are in living organisms.
Bibliography
Blum, Kenneth, and Ernest P. Noble, eds. Handbook of Psychiatric Genetics. New York:CRC Press, 1996.
Ezell, Carol. "Beyond the Human Genome." Scientific American 283 (2000): 64-69.
Mange, Arthur P., and Elaine J. Mange. Genetics: Human Aspects. Sunderland, MA:Sinauer Associates, 1990.
Race, Robert R., and Ruth Sanger. Blood Groups in Man, 6th ed. Oxford: BlackwellScientific Publications, 1975.
—P. Michael Conneally
| Veterinary Dictionary: epistasis |
Non-allelic masking of one gene by another, e.g. the masking of the black gene by the orange gene in tortoiseshell cats.
| Wikipedia: Epistasis |
Epistasis is the interaction between genes. Epistasis takes place when the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is said to be epistatic, while the phenotype altered or suppressed is said to be hypostatic. Epistasis can be contrasted with dominance, which is an interaction between alleles at the same gene locus. Epistasis is often studied in relation to Quantitative Trait Loci (QTL) and polygenic inheritance.
In general, the fitness increment of any one allele depends in a complicated way on many other alleles; but, because of the way that the science of population genetics was developed, evolutionary scientists tend to think of epistasis as the exception to the rule. In the first models of natural selection devised in the early 20th century, each gene was considered to make its own characteristic contribution to fitness, against an average background of other genes. Some introductory college courses still teach population genetics this way.
Epistasis and genetic interaction refer to different aspects of the same phenomenon. The term epistasis is widely used in population genetics and refers especially to the statistical properties of the phenomenon, and does not necessarily imply biochemical interaction between gene products. However, in general epistasis is used to denote the departure from 'independence' of the effects of different genetic loci. Confusion often arises due to the varied interpretation of 'independence' between different branches of biology. For further discussion of the definitions of epistasis, and the history of these definitions, see [1].
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.
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.
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Two-locus epistatic interactions can be either synergistic (enhancing the effectiveness) or antagonistic (reducing the activity).[2][3] In the example of a haploid organism with genotypes (at two loci) AB, Ab, aB or ab, we can think of the following trait values where higher values suggest greater expression of the characteristic (the exact values are simply given as examples):
| AB | Ab | aB | ab | |
| No epistasis (additive across loci) | 2 | 1 | 1 | 0 |
| Synergistic epistasis | 3 | 1 | 1 | 0 |
| Antagonistic epistasis | 1 | 1 | 1 | 0 |
Hence, we can classify thus:
| Trait values | Type of epistasis |
| AB = Ab + aB - ab | No epistasis, additive inheritance |
| AB > Ab + aB - ab | Synergistic epistasis |
| AB < Ab + aB - ab | Antagonistic epistasis |
Understanding whether the majority of genetic interactions are synergistic or antagonistic will help solve such problems as the evolution of sex.
Negative epistasis and sex are thought to be intimately correlated. Experimentally, this idea has been tested in using digital simulations of asexual and sexual populations. Over time, sexual populations move towards more negative epistasis, or the lowering of fitness by two interacting alleles. It is thought that negative epistasis allows individuals carrying the interacting deleterious mutations to be removed from the populations efficiently. This removes those alleles from the population, resulting in an overall more fit population. This hypothesis was proposed by Alexey Kondrashov, and is sometimes known as the deterministic mutation hypothesis[4] and has also been tested using artificial gene networks.[2]
However, the evidence for this hypothesis has not always been straightforward and the model proposed by Kondrashov has been criticized for assuming mutation parameters far from real world observations. For example, see [5]. In addition, in those tests which used artificial gene networks, negative epistasis is only found in more densely connected networks[2], whereas empirical evidence indicates that natural gene networks are sparsely connected[6], and theory shows that selection for robustness will favor more sparsely connected and minimally complex networks.[6]
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 | Blood Type |
| Complex Traits |
| Inheritance Patterns |
 | Which of the following provides an example of epistasis? |
| What happens during epistasis? |
| What are classical examples of Epistasis and describe? |
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Genetics Encyclopedia. Genetics. Copyright © 2003 by The Gale Group, Inc. All rights reserved. Read more |
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| Veterinary Dictionary. Saunders Comprehensive Veterinary Dictionary 3rd Edition. Copyright © 2007 by D.C. Blood, V.P. Studdert and C.C. Gay, Elsevier. All rights reserved. Read more |
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| Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Epistasis". Read more |
