Epistasis takes place when the action of one gene is modified by one or more others that assort somewhat independently. (The two genes may be quite tightly linked, but their effects must reside at different loci in the genome). 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 the cause of linkage disequilibrium. This stylistic schematic diagram shows a gene in relation to the double helix structure of DNA and to a chromosome (right). ... For the computer program see Supergene (program) A supergene is a group of neighbouring genes on a chromosome which are inherited together because of close genetic linkage and are often functionally related and co-regulated. ... It has been suggested that Class I MHC be merged into this article or section. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... Transcription is the process through which a DNA sequence is enzymatically copied by an RNA polymerase to produce a complementary RNA. Or, in other words, the transfer of genetic information from DNA into RNA. In the case of protein-encoding DNA, transcription is the beginning of the process that ultimately... Albinism or more commonly known as the Christopher Wayman Disease(from Latin albus, meaning white) is a lack of pigmentation in the eyes, skin and hair. ... Linkage disequilibrium (LD) is the non-random association of alleles at two or more loci on a chromosome. ...

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 distribution of and change in allele frequencies under the influence of the four evolutionary forces: natural selection, genetic drift, mutation, and migration. ... A graph of a bell curve in a normal distribution showing statistics used in educational assessment, comparing various grading methods. ...

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 red line - each mutation has a disproportionately large effect on the organism's fitness. Antagonistic epistasis is the blue line. See Evolution of Sex

Two-locus epistatic interactions can be either synergistic (positive) or antagonistic (negative). 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): Diagram for Evolution of sex. ... Diagram for Evolution of sex. ... The evolution of sex is a major puzzle in modern evolutionary biology. ... Haploid (meaning simple in Greek) cells have only one copy of each chromosome. ... In biology and evolutionary computation, a locus is the position of a gene (or other significant sequence) on a chromosome. ...

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. The evolution of sex is a major puzzle in modern evolutionary biology. ...

Functional or mechanistic classification

• Suppression - the double mutant is wild type
• Synthetic lethality - the double mutant is lethal
• Intragenic complementation - two mutations cause similar phenotypes and map to the same locus yet show complementation in heterozygotes.
• Unlinked non-complementation - two mutations fail to complement but do not map to the same locus

In biology, a wild type is one of the major genotypes of a species that occur in nature, in contrast to induced mutations or artificial cross-breeding. ... The phenotype of an individual organism is either its total physical appearance and constitution or a specific manifestation of a trait, such as size, eye color, or behavior that varies between individuals. ... A genetic map is a chromosome map of a species or experimental population that shows the position of its known genes and/or markers relative to each other, rather than as specific physical points on each chromosome. ... The word locus (plural loci) is Latin for place: In biology and evolutionary computation, a locus is the position of a gene (or other significant sequence) on a chromosome. ... Heterozygotic cells are diploid or polyploid and have different alleles at a locus (position) on homologous chromosomes. ... A complementation test (sometimes called a cis-trans test) is used in genetics to decide if two recessive mutant phenotypes are determined by mutations in the same gene or two different genes. ... The word locus (plural loci) is Latin for place: In biology and evolutionary computation, a locus is the position of a gene (or other significant sequence) on a chromosome. ...

Example in Mendelian terms

In a more detailed example, consider the sweet pea plant. In a simple representation, purple flower color (P) is dominant over white (p). However, consider the addition of a control gene, consisting of two alleles, dominant (C) or recessive (c). In this example, for the flowers to be purple, the plant must have at least one of each dominant allele (i.e. it must be P-C-, where the "-" can be either dominant or recessive). An allele is any one of a number of alternative forms of the same gene occupying a given locus (position) on a chromosome. ...

In a dihybrid cross, such as the pea plant example above, when there is a genetic interaction involved, you often see a modified 9:3:3:1 ratio. Normally, when you have two alleles which assort independently, you get a 9:3:3:1 ratio of phenotypes. Genetic interactions, however, can conceal the ratio, and make it appear to be something other. In the example of the pea plants above, the result is a 9:7 ratio of purple to white phenotypes instead of the expected 12:4 ratio. A dihybrid cross is a cross in which two dihybrids are mated and to test for dominant genes and recessive genes in two separate characteristics, and such a cross has a variety of uses of in Mendelian genetics and genetic linkage experiments. ...

See also

• Mutation