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Quantitative genetics
 
 
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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
Mendelian inheritance

Mendelian inheritance is a set of primary tenets relating to the transmission of heredity characteristics from parent organisms to their children; it underlies much of genetics....
 in that the combined effect of the many underlying genes results in a continuous distribution
Continuous probability distribution

In probability theory, a probability distribution is called continuous if its cumulative distribution function is continuous function. This is equivalent to saying that for random variables X with the distribution in question, Pr[X = a] = 0 for all real numbers a, i.e.: the probability that X attains the value a is zer...
 of phenotypic values.

field was founded, in evolution
Evolution

In biology, evolution is change in the heritability trait of a population of organisms from one generation to the next. These changes are caused by a combination of three main processes: variation, reproduction, and selection....
ary terms, by the originators of the modern synthesis
Modern evolutionary synthesis

The modern evolutionary synthesis is a union of ideas from several biology specialties which forms a logical account of evolution. This synthesis has been generally accepted by most working biologists....
, R.A. Fisher
Ronald Fisher

Sir Ronald Aylmer Fisher, Fellow of the Royal Society was an England statistician, evolutionary biologist, and genetics. He was described by Anders Hald as "a genius who almost single-handedly created the foundations for modern statistical science" and Richard Dawkins described him as "the greatest of Charles Darwin successors"....
, Sewall Wright
Sewall Wright

Sewall Green Wright was an American geneticist known for his influential work on evolutionary theory and also for his work on path analysis . With R....
 and J. B. S. Haldane
J. B. S. Haldane

John Burdon Sanderson Haldane Royal Society#Fellowship , known as Jack , was a UK-born geneticist and evolutionary biologist. He was one of the founders of population genetics....
, and aimed to predict the response to selection given data on the phenotype
Phenotype

A phenotype is any observable characteristic or trait_ of an organism: such as its morphology , development, biochemical or physiological properties, or behavior....
 and relationships of individuals.

Analysis of Quantitative trait loci
Quantitative trait locus

Inheritance of quantitative traits or polygenic inheritance refers to the inheritance of a phenotype characteristic that varies in degree and can be attributed to the interactions between two or more genes and their environment....
, or QTL, is a more recent addition to the study of quantitative genetics.






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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
Mendelian inheritance

Mendelian inheritance is a set of primary tenets relating to the transmission of heredity characteristics from parent organisms to their children; it underlies much of genetics....
 in that the combined effect of the many underlying genes results in a continuous distribution
Continuous probability distribution

In probability theory, a probability distribution is called continuous if its cumulative distribution function is continuous function. This is equivalent to saying that for random variables X with the distribution in question, Pr[X = a] = 0 for all real numbers a, i.e.: the probability that X attains the value a is zer...
 of phenotypic values.

History

The field was founded, in evolution
Evolution

In biology, evolution is change in the heritability trait of a population of organisms from one generation to the next. These changes are caused by a combination of three main processes: variation, reproduction, and selection....
ary terms, by the originators of the modern synthesis
Modern evolutionary synthesis

The modern evolutionary synthesis is a union of ideas from several biology specialties which forms a logical account of evolution. This synthesis has been generally accepted by most working biologists....
, R.A. Fisher
Ronald Fisher

Sir Ronald Aylmer Fisher, Fellow of the Royal Society was an England statistician, evolutionary biologist, and genetics. He was described by Anders Hald as "a genius who almost single-handedly created the foundations for modern statistical science" and Richard Dawkins described him as "the greatest of Charles Darwin successors"....
, Sewall Wright
Sewall Wright

Sewall Green Wright was an American geneticist known for his influential work on evolutionary theory and also for his work on path analysis . With R....
 and J. B. S. Haldane
J. B. S. Haldane

John Burdon Sanderson Haldane Royal Society#Fellowship , known as Jack , was a UK-born geneticist and evolutionary biologist. He was one of the founders of population genetics....
, and aimed to predict the response to selection given data on the phenotype
Phenotype

A phenotype is any observable characteristic or trait_ of an organism: such as its morphology , development, biochemical or physiological properties, or behavior....
 and relationships of individuals.

Analysis of Quantitative trait loci
Quantitative trait locus

Inheritance of quantitative traits or polygenic inheritance refers to the inheritance of a phenotype characteristic that varies in degree and can be attributed to the interactions between two or more genes and their environment....
, 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.

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 often polygenic, and may also be influenced significantly by environmental effects.
  • Meristic traits or other ordinal number
    Ordinal number

    In set theory, an ordinal number, or just ordinal, is the order type of a well-order. They are usually identified with hereditarily transitive sets....
    s are expressed in whole number
    Whole number

    The term whole number is used by various authors to mean either:*the nonnegative integer *the positive integer *all integer ...
    s, such as number of offspring, or number of bristles on a fruit fly
    Fruit fly

    Fruit fly may refer to:* Tephritidae, the family of large fruit flies.* Drosophilidae, the family of small fruit flies and vinegar flies, including:...
    . 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
    Schizophrenia

    Schizophrenia , from the Ancient Greek Root schizein and phren, phren- is a psychiatry diagnosis that describes a mental disorder characterized by abnormalities in the perception or expression of reality....
    ) have been studied in this manner.


Basic principles


The phenotypic value (P) of an individual
Individual

As vernacular, individual refers to a person or to any specific object in a collection. In the 15th century and earlier, and also today within the fields of statistics and metaphysics, individual means "indivisible", typically describing any numerically singular thing, but sometimes meaning "a person." ....
 is the combined effect of the genotypic value (G) and the environmental deviation (E):
P = G + E
The genotypic value is the combined effect of all the genetic effects, including nuclear gene
Nuclear gene

Nuclear gene is a gene located in the cell nucleus of an eukaryote. The term is used to distinguish nuclear genes from the genes in the mitochondrion, or in case of plants, also the chloroplast, which host their own genetic system and can produce proteins from scratch....
s, mitochondrial
Mitochondrion

In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryote cell . These organelles range from 0.5–10 micrometers in diameter....
 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
Gene-environment interaction

Gene?environment interaction, also called genotype?environment interaction or GxE, is a term used to describe any Phenotype effects that are due to interactions between the environment and genes....
. This can be described as:
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 variance
Variance

In probability theory and statistics, the variance of a random variable, probability distribution, or sample is one measure of statistical dispersion, averaging the squared distance of its possible values from the expected value ....
s for those components, denoted as:
VP = VA + VD + VE + VI
The heritability
Heritability

In genetics, Heritability is the proportion of phenotype in a population that is attributable to genotype among individuals. Variation among individuals may be due to genetic and/or environmental factors....
 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
Natural selection

Natural selection is the process by which favorable heritable trait become more common in successive generations of a population of Reproduction organisms, and unfavorable heritable traits become less common, due to differential reproduction of genotypes....
 or artificial selection
Artificial selection

Artificial selection describes intentional breeding for certain traits, or combination of traits. It was defined by Charles Darwin in contrast to natural selection, in which the differential reproduction of organisms with certain traits is attributed to improved survival or reproductive ability ....
.

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 alleles, 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 alleles with each other, while half sibs share only 25% of their alleles. This variation in relatedness can be used to estimate which proportion of the total phenotypic variance (VP) is explained by the above-mentioned components.

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 covariance
Covariance

In probability theory and statistics, covariance is a measure of how much two variables change together .If two variables tend to vary together , then the covariance between the two variables will be positive....
s, and the phenotypic covariance (CovP) between two traits can be partitioned in the same way as the variances described above. The genetic correlation
Genetic correlation

Genetic correlation refers to the proportion of variance that two traits share due to genetic causes. Outside the theoretical boundary case of traits with zero heritability, the genetic correlation of traits is independent of their heritability: i.e., two traits can have a very high genetic correlation even when the heritability of each is...
 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:

See also

  • Ewens's sampling formula
    Ewens's sampling formula

    In population genetics, Ewens' sampling formula, introduced by Warren Ewens, states that under certain conditions , 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 in the sample, and a
  • Heritability
    Heritability

    In genetics, Heritability is the proportion of phenotype in a population that is attributable to genotype among individuals. Variation among individuals may be due to genetic and/or environmental factors....
  • Artificial selection
    Artificial selection

    Artificial selection describes intentional breeding for certain traits, or combination of traits. It was defined by Charles Darwin in contrast to natural selection, in which the differential reproduction of organisms with certain traits is attributed to improved survival or reproductive ability ....
  • Selective breeding
    Selective breeding

    Selective breeding in domesticated animals is the process of a Breeder developing a cultivated breed over time, and selecting qualities within individuals of the breed that will be best to pass on to the next generation....
  • Experimental evolution
    Experimental evolution

    In evolutionary biology, the field of experimental evolution is concerned with testing hypotheses and evolution by use of controlled experiments....
  • Population genetics
    Population genetics

    Population genetics is the study of the allele frequency distribution and change under the influence of the four evolutionary processes: natural selection, genetic drift, mutation and gene flow....
    , built on some of the same principles as quantitative genetics
  • Genetic distance
    Genetic distance

    Genetic distance is a measure of the dissimilarity of genetic material between different species or individuals of the same species. By comparing the percentage difference between the same genes or junk DNA of different species, a figure can be obtained, which is a measure of "genetic distance"....


External links

  • by Michael Lynch
    Michael Lynch (geneticist)

    Michael Lynch is Distinguished Professor of Evolution, Population Genetics and Genomics at Indiana University Bloomington. Besides many highly acclaimed papers, especially in population genetics, he has written a two volume textbook with Bruce Walsh , widely considered the "Bible" of quantitative genetics....
     and Bruce Walsh, including the two volumes of their textbook, Genetics and Analysis of Quantitative Traits and Evolution and Selection of Quantitative Traits.
  • Nick Barton
    Nick Barton

    Professor Nicholas Hamilton Barton Fellow of the Royal Society Fellow of the Royal Society of Edinburgh is a United Kingdom evolutionary biologist....
     et al. from the textbook, Evolution