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Quantitative genetics is the study of continuous traits (such as height or weight) and its underlying mechanisms. It is effectively an extension of simple Mendelian inheritance in that the combined effect of the many underlying genes results in a continuous distribution of phenotypic values. Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent organisms to their children; it underlies much of genetics. ...
The phenotype of an individual organism is either its total physical appearance and constitution, or a specific manifestation of a trait, such as size or eye color, that varies between individuals. ...
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History
The field was founded, in evolutionary terms, by the originators of the modern synthesis, R.A. Fisher, Sewall Wright and J. B. S. Haldane, and aimed to predict the response to selection given data on the phenotype and relationships of individuals. A phylogenetic tree of all extant organisms, based on 16S rRNA gene sequence data, showing the evolutionary history of the three domains of life, bacteria, archaea and eukaryotes. ...
The modern evolutionary synthesis (often referred to simply as the modern synthesis), neo-Darwinian synthesis or neo-Darwinism, brings together Charles Darwins theory of the evolution of species by natural selection with Gregor Mendels theory of genetics as the basis for biological inheritance. ...
Sir Ronald Aylmer Fisher, FRS (17 February 1890 – 29 July 1962) was a English statistician, evolutionary biologist, and geneticist. ...
Sewall Green Wright ForMemRS (December 21, 1889 – March 3, 1988) was an American geneticist known for his influential work on evolutionary theory. ...
John Burdon Sanderson Haldane (November 5, 1892 – December 1, 1964), who normally used J.B.S. as a first name, was a British geneticist and evolutionary biologist. ...
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. ...
Analysis of Quantitative trait loci, or QTL, is a more recent addition to the study of quantitative genetics. A QTL is a region in the genome that affects the trait or traits of interest. Quantitative trait loci approaches require accurate phenotypic, pedigree and genotypic data from a large number of individuals. Quantitative trait locus (QTL) is a region of DNA that is associated with a particular trait (e. ...
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Traits Quantitative genetics is not limited to continuous traits, but to all traits that are determined by many genes. This includes: - Continuous traits are quantitative traits with a continuous phenotypic range. They are usually polygenic, and may also have a significant environmental influence.
- Meristic trait or other ordinal numbers are expressed in whole numbers, such as number of offspring, or number of bristles on a fruit fly. These traits can be either treated as approximately continuous traits or as threshold traits.
- Some qualitative traits can be treated as if they have an underlying quantitative basis, expressed as a threshold trait (or multiple thresholds). Some human diseases (such as, schizophrenia) have been studied in this manner.
[edit] The whole numbers are the nonnegative integers (0, 1, 2, 3, ...) The set of all whole numbers is represented by the symbol = {0, 1, 2, 3, ...} Algebraically, the elements of form a commutative monoid under addition (with identity element zero), and under multiplication (with identity element one). ...
Basic principles The phenotypic value (P) of an individual is the combined effect of the genotypic value (G) and the environmental deviation (E): In common speech, the word individual most often refers to a person, or, by analogy, to any specific object in a group of things. ...
- P = G + E
The genotypic value is the combined effect of all the genetic effects, including nuclear genes, mitochondrial genes and interactions between the genes. It is therefore often subdivided in an additive (A) and a dominance component (D). The additive effect described the cumulative effect of the individual genes, while the dominance effect is the result of interactions between those genes. The environmental deviation can be subdivided in a pure environmental component (E) and an interaction factor (I) describing the interaction between genes and the environment. This can be described as: Nuclear gene is a gene located in the cell nucleus of an eucaryote. ...
Electron micrograph of a mitochondrion showing its mitochondrial matrix and membranes In cell biology, a mitochondrion (plural mitochondria) (from Greek μιτος or mitos, thread + κουδÏιον or khondrion, granule) is an organelle, variants of which are found in most eukaryotic cells. ...
In biology and psychology Gene-environment interaction is a term used to describe any phenotypic effects that are due to interactions between the environment and genes. ...
- P = A + D + E + I
The contribution of those components cannot be determined in a single individual, but they can be estimated for whole populations by estimating the variances for those components, denoted as: In probability theory and statistics, the variance of a random variable is a measure of its statistical dispersion, indicating how far from the expected value its values typically are. ...
- VP = VA + VD + VE + VI
The heritability of a trait is the proportion of the total (i.e. phenotypic) variation (VP) that is explained by the genetic variation. This is the total genetic variation (VG) in broad sense heritabilities (H2), while only the additive genetic variation (VA) is used for narrow sense heritabilities (h2), often simply called heritability. The latter gives an indication how a trait will respond to natural or artificial selection. Heritability, as used professionally in genetics, has a very precise definition. ...
Natural selection is the process by which individual organisms with favorable traits are more likely to survive and reproduce. ...
This Chihuahua mix and Great Dane show the wide range of dog breed sizes created using artificial selection. ...
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Resemblance between relatives Central in estimating the variances for the various components is the principle of relatedness. A child has a father and a mother. Consequently, the child and father share 50% of their genes, as do the child and the mother. However, the mother and father normally do not share genes as a result of shared ancestors. Similarly, two full siblings share also on average 50% of the genes with each other, while half sibs share only 25% of the genes. This variation in relatedness can be used to estimate which proportion of the total phenotypic variance (VP) is explained by the above-mentioned components. [edit]
Correlated traits Although some genes have only an effect on a single trait, many genes have an effect on various traits. Because of this, a change in a single gene will have an effect on all those traits. This is calculated using covariances, and the phenotypic covariance (CovP) between two traits can be partitioned in the same way as the variances described above. The genetic correlation is calculated by dividing the covariance between the additive genetic effects of two traits by the square root of the product of the variances for the additive genetic effects of the two traits: In probability theory and statistics, the covariance between two real-valued random variables X and Y, with expected values and is defined as: where E is the expected value. ...
Linear correlations between 1000 pairs of numbers. ...
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References - Falconer, D. S. & Mackay TFC (1996). Introduction to Quantitative Genetics. Fourth edition. Addison Wesley Longman, Harlow, Essex, U.K.
- Roff DA (1997). Evolutionary Quantitative Genetics. Chapman & Hall, New York.
- Lynch M & Walsh B (1998). Genetics and Analysis of Quantitative Traits. Sinauer, Sunderland, MA.
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See also [edit] In population genetics, Ewenss sampling formula, introduced by Warren Ewens, states that under certain conditions (specified below), if a random sample of n gametes is taken from a population and classified according to the gene at a particular locus then the probability that there are a1 alleles represented once...
Heritability, as used professionally in genetics, has a very precise definition. ...
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. ...
External links - Quantitative Genetics Resources by Michael Lynch and Bruce Walsh, including the two volumes of their textbook, Genetics and Analysis of Quantitative Traits and Evolution and Selection of Quantitative Traits.
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