Gene–environment interaction - Wikipedia
Epistasis is the term that refers to the action of one gene upon another. obtained an F2 generation with a phenotypic ratio of , as summarized in Table 1. The term gene interaction has also been used with reference to direct as a tool to dissect the functional relationships among sets of genes (Guarente, ;. This phenomenon, which defines genetic interaction, can reveal functional relationships between genes and pathways. For example, double.
Thus all females had red eyes like their father.
Since the male fruit fly passes only the Y to his sons, their eye color is determined entirely by the single X chromosome they receive from their mother in this case white.
Thus all the males in the second cross were white eyed. These experiments introduced the concept of sex-linkagethe occurrence of genes on that part of the X that lack a corresponding location on the Y. Sex-linked recessives such as white eyes in fruit flies, hemophiliabaldness, and colorblindness in humans occur more commonly in males, since there is no chance of them being heterozygous. Such a condition is termed hemizygous. Inheritance of eye color in fruit flies.
Images from Purves et al. Characteristics of X-linked Traits 1. Phenotypic expression more common in males 2. Sons cannot inherit the trait from their fathers, but daughters can. Sons inherit their Y chromosome from their father. Only a few genes have been identified on the Y chromosome, among them the testis-determining factor TDF that promotes development of the male phenotype. Barr bodies are interpreted as inactivated X chromosomes in mammalian females.
Since females have two X chromosomes, the Lyon hypothesis suggests that one or the other X is inactivated in each somatic non-reproductive cell during embryonic development. Cells mitotically produced from these embryonic cells likewise have the same inactivated X chromosome.
The role of deactivated X chromosomes in mammalian female development. Barr bodies b in mammalian cells. The above image is from http: Calico cats sometimes called tortoiseshell are almost always female since the calico trait is caused by some areas of the cat's fur expressing one allele and others expressing the other color.
Can there be a male tortoiseshell cat? How would such a cat get its genes? Remember that fur color in cats is a sex-linked feature. Would the male calico be fertile or sterile?
These proteins are responsible for the expression of the phenotype. The basic principles of segregation and independent assortment as worked out by Mendel are applicable even for sex-linked traits. Codominant alleles Codominant alleles occur when rather than expressing an intermediate phenotype, the heterozygotes express both homozygous phenotypes.
Blood Type A people manufacture only anti-B antibodies, while type B people make only anti-A antibodies. Codominant alleles are both expressed. Heterozygotes for codominant alleles fully express both alleles. Blood type AB individuals produce both A and B antigens. Since neither A nor B is dominant over the other and they are both dominant over O they are said to be codominant. Incomplete dominance Incomplete dominance is a condition when neither allele is dominant over the other.
The condition is recognized by the heterozygotes expressing an intermediate phenotype relative to the parental phenotypes. If a red flowered plant is crossed with a white flowered one, the progeny will all be pink. When pink is crossed with pink, the progeny are 1 red, 2 pink, and 1 white. Inheritance of flower color in snapdragons. Image from Purves et al. Flower color in snapdragons is an example of this pattern. Cross a true-breeding red strain with a true-breeding white strain and the F1 are all pink heterozygotes.
Self-fertilize the F1 and you get an F2 ratio of 1 red: This would not happen if true blending had occurred blending cannot explain traits such as red or white skipping a generation and pink flowers crossed with pink flowers should produce ONLY pink flowers. Multiple alleles Many genes have more than two alleles even though any one diploid individual can only have at most two alleles for any genesuch as the ABO blood groups in humans, which are an example of multiple alleles.
Multiple alleles result from different mutations of the same gene. Coat color in rabbits is determined by four alleles. A and B are codominants which are both dominant over O.
The only possible genotype for a type O person is OO. Type AB have only the AB heterozygous genotype. The A and B alleles of gene I produce slightly different glycoproteins antigens that are on the surface of each cell. Homozygous A individuals have only the A antigen, homozygous B individuals have only the B antigen, homozygous O individuals produce neither antigen, while a fourth phenotype AB produces both A and B antigens.
Interactions among genes Back to Top While one gene may make only one protein, the effects of those proteins usually interact for example widow's peak may be masked by expression of the baldness gene. Novel phenotypes often result from the interactions of two genes, as in the case of the comb in chickens.
The single comb is produced only by the rrpp genotype. Bateson reported a different phenotypic ratio in sweet pea than could be explained by simple Mendelian inheritance. This ratio is 9: Of the two genes C and Pwhen either is homozygous recessive cc or pp that gene is epistatic to or hides the other. To get purple flowers one must have both C and P alleles present.
In buttercup Ranunculus peltatusleaves below water-level are finely divided and those above water-level are broad, floating, photosynthetic leaf-like leaves. Siamese cats are darker on their extremities, due to temperature effects on phenotypic expression.
Expression of phenotype is a result of interaction between genes and environment. Siamese cats and Himalayan rabbits both animals have dark colored fur on their extremities.
This is caused by an allele that controls pigment production being able only to function at the lower temperatures of those extremities. Environment determines the phenotypic pattern of expression.
Polygenic Inheritance Back to Top Polygenic inheritance is a pattern responsible for many features that seem simple on the surface. Many traits such as height, shape, weight, color, and metabolic rate are governed by the cumulative effects of many genes. Polygenic traits are not expressed as absolute or discrete characters, as was the case with Mendel's pea plant traits. Instead, polygenic traits are recognizable by their expression as a gradation of small differences a continuous variation.
The results form a bell shaped curve, with a mean value and extremes in either direction.
Height in humans is a polygenic trait, as is color in wheat kernels. Conversely, if mutations interact with one another by epistasis, the fitness landscape becomes rugged as the effect of a mutation depends on the genetic background of other mutations.
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This is referred to as a rugged fitness landscape and has profound implications for the evolutionary optimisation of organisms.
If mutations are deleterious in one combination but beneficial in another, the fittest genotypes can only be accessed by accumulating mutations in one specific order. This makes it more likely that organisms will get stuck at local maxima in the fitness landscape having acquired mutations in the 'wrong' order.
In contrast, changes in environment and therefore the shape of the fitness landscape have been shown to provide escape from local maxima.
This gateway mutation alleviated the negative epistatic interactions of other individually beneficial mutations, allowing them to better function in concert.
Complex environments or selections may therefore bypass local maxima found in models assuming simple positive selection. High epistasis is usually considered a constraining factor on evolution, and improvements in a highly epistatic trait are considered to have lower evolvability. This is because, in any given genetic background, very few mutations will be beneficial, even though many mutations may need to occur to eventually improve the trait.
The lack of a smooth landscape makes it harder for evolution to access fitness peaks. In highly rugged landscapes, fitness valleys block access to some genes, and even if ridges exist that allow access, these may be rare or prohibitively long. Rugged, epistatic fitness landscapes also affect the trajectories of evolution.
When a mutation has a large number of epistatic effects, each accumulated mutation drastically changes the set of available beneficial mutations.Gene environment interaction - Behavior - MCAT - Khan Academy
Therefore, the evolutionary trajectory followed depends highly on which early mutations were accepted. Thus, repeats of evolution from the same starting point tend to diverge to different local maxima rather than converge on a single global maximum as they would in a smooth, additive landscape.
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 Kondrashovand is sometimes known as the deterministic mutation hypothesis  and has also been tested using artificial gene networks.
Any two locus interactions at a particular gene frequency can be decomposed into eight independent genetic effects using a weighted regression.
Epistasis - Wikipedia
In this regression, the observed two locus genetic effects are treated as dependent variables and the "pure" genetic effects are used as the independent variables. Because the regression is weighted, the partitioning among the variance components will change as a function of gene frequency.
By analogy it is possible to expand this system to three or more loci, or to cytonuclear interactions  Double mutant cycles[ edit ] When assaying epistasis within a gene, site-directed mutagenesis can be used to generate the different genes, and their protein products can be assayed e.
This is sometimes called a double mutant cycle and involves producing and assaying the wild type protein, the two single mutants and the double mutant. Epistasis is measured as the difference between the effects of the mutations together versus the sum of their individual effects. The same methodology can be used to investigate the interactions between larger sets of mutations but all combinations have to be produced and assayed.
For example, there are different combinations of 5 mutations, some or all of which may show epistasis