Epistasis is the interaction between genes. Interaction is a kind of action that occurs as two or more objects have an Effect upon one another Epistasis takes place when the action of one gene is modified by one or several other genes, which are sometimes called modifier genes. History See also History of genetics The existence of genes was first suggested by Gregor Mendel (1822-1884 who in the 1860s studied inheritance The gene whose phenotype is expressed is said to be epistatic, while the phenotype altered or suppressed is said to be hypostatic. A phenotype is any observable characteristic of an Organism, such as its morphology, Development, biochemical or physiological properties

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. Population genetics is the study of the Allele frequency distribution and change under the influence of the four evolutionary forces Natural selection, Genetic 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. Natural selection is the process by which favorable Heritable traits become more common in successive Generations of a Population of In introductory college courses, population genetics is still taught this way. Population genetics is the study of the Allele frequency distribution and change under the influence of the four evolutionary forces Natural selection, Genetic

Epistasis and genetic interaction refer to the same phenomenon; however, epistasis is widely used in population genetics and refers especially to the statistical properties of the phenomenon. Population genetics is the study of the Allele frequency distribution and change under the influence of the four evolutionary forces Natural selection, Genetic Statistics is a mathematical science pertaining to the collection analysis interpretation or explanation and presentation of Data.

Examples of tightly linked genes having epistatic effects on fitness are found in supergenes and the human major histocompatibility complex genes. A supergene is a group of neighbouring Genes on a Chromosome which are inherited together because of close Genetic linkage and are functionally related The major histocompatibility complex ( MHC) is a large genomic region or Gene family found in most Vertebrates It is the most gene-dense region The effect can occur directly at the genomic level, where one gene could code for a protein preventing transcription of the other gene. Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Transcription is the synthesis of RNA under the direction of DNA 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. Albinism (from Latin albus, "white" see extended etymology) is a form of hypopigmentary Congenital disorder, In another example, a gene coding for a widow's peak would be hidden by a gene causing baldness. A widow's peak (widow's brow is a descending V-shaped point in the middle of the hairline (above the forehead Fitness epistasis (where the affected trait is fitness) is one cause of linkage disequilibrium. Fitness (often denoted w in Population genetics models is a central concept in evolutionary theory. In Population genetics, linkage disequilibrium is the non-random association of Alleles at two or more loci, not necessarily on the same Chromosome

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.

Classification by fitness or trait value

Diagram illustrating different relationships between numbers of mutations and fitness. Synergistic epistasis is the blue line - each mutation has a disproportionately large effect on the organism's fitness. Antagonistic epistasis is the red line. See Evolution of Sex

Two-locus epistatic interactions can be either synergistic (negative) or antagonistic (positive). ^[1] In the example of a haploid organism with genotypes (at two loci) AB, Ab, aB and 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):

Hence, we can classify thus:

Understanding whether the majority of genetic interactions are synergistic or antagonistic will help solve such problems as the evolution of sex. "Haplo" redirects here For the fictional character see The Death Gate Cycle. In the fields of Genetics and Evolutionary computation, a locus (plural loci) is a fixed position on a Chromosome such as the position of a

Epistasis and 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^[2] and has also been tested using artificial gene networks. Alexey S Kondrashov is a professor at the University of Michigan in Ann Arbor MI. ^[3]

However, the evidence for this hypothesis has not always been straightforward and the model proposed by Kondrashov has often been criticized for assuming mutation parameters far from real world observations. For example, see ^[4]

Functional or mechanistic classification

• Genetic suppression - the double mutant has a less severe phenotype than either single mutant.
• Genetic enhancement - the double mutant has a more severe phenotype than one predicted by the additive effects of the single mutants.
• Synthetic lethality or unlinked non-complementation - two mutations fail to complement and yet do not map to the same locus. In Genetics, complementation refers to a relationship between two different strains of an organism which both have homozygous recessive Mutations In the fields of Genetics and Evolutionary computation, a locus (plural loci) is a fixed position on a Chromosome such as the position of a
• Intragenic complementation, allelic complementation, or interallelic complementation - two mutations map to the same locus, yet the two alleles complement in the heteroallelic diploid. Genetic linkage occurs when particular genetic loci or Alleles for genes are inherited jointly Causes of intragenic complementation include:
  • homology effects such as transvection, where, for example, an enhancer from one allele acts in trans to activate transcription from the promoter of the second allele. Transvection is an Epigenetic phenomenon that results from an interaction between an Allele on one Chromosome and the corresponding allele on the homologous
  • trans-splicing of two mutant RNA molecules to produce a functional RNA.
  • At the protein level, another possibility involves proteins that normally function as dimers. In a heteroallelic diploid, two different abnormal proteins could form a functional dimer if each can compensate for the lack of function in the other.

See also

• Co-adaptation
• Mutation

References

1. ^ Azevedo R, Lohaus R, Srinivasan S, Dang K, Burch C (2006). In Biology, co-adaptation, or coadaptation refers to the mutual adaptation of Species: see Mutualism, Symbiosis In biology mutations are changes to the Nucleotide sequence of the Genetic material of an organism "Sexual reproduction selects for robustness and negative epistasis in artificial gene networks". Nature 440 (7080): 87-90. doi:10.1038/nature04488. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 16511495.  
2. ^ A. S. Kondrashov (1988). "Deleterious mutations and the evolution of sexual reproduction". Nature 336: 435-440. Nature is a prominent Scientific journal, first published on 4 November 1869  
3. ^ Azevedo R, Lohaus R, Srinivasan S, Dang K, Burch C (2006). "Sexual reproduction selects for robustness and negative epistasis in artificial gene networks". Nature 440 (7080): 87-90. Nature is a prominent Scientific journal, first published on 4 November 1869 PMID 16511495.  
4. ^ MacCarthy T, Bergman A. (July 2007). "Coevolution of robustness, epistasis, and recombination favors asexual reproduction". Proc Natl Acad Sci U S A 104 (31): 12801-6. The Proceedings of the National Academy of Sciences of the United States of America, usually referred to as PNAS, is the official journal of the United  

External links

• GeneticInteractions.org
• Epistasis.org