Polygene
Encyclopedia
A polygene, multiple factor, multiple gene inheritance, or quantitative gene is a group of non-allelic gene
s that together influence a phenotypic
trait. The loci or identities of the non-allelic genes are often unknown to biologist. Advances in statistical methodology and high throughput sequencing are, however, allowing researchers to locate candidate genes for the trait. These genes are generally pleiotropic
as well. The genes that contribute to type 2 diabetes are thought to be mostly polygenes.
s. Often the genes are large in quantity but small in effect. Examples of human polygenic inheritance are height, skin color and weight. Polygenes exist in other organisms, as well. Drosophila
, for instance, display polygeny with traits such as wing morphology, bristle count and many others.
of the distribution represents the optimal, or fittest, phenotype. The more genes are involved, the smoother the estimated curve. However, in this model all genes must code for alleles with additive
effects. This assumption is often unrealistic as many genes display epistasis effects which can have unpredictable affects on the distribution of outcomes, especially when looking at the distribution on a fine scale.
Traditionally, mapping polygenes requires statistical tools available to help measure the effects of polygenes as well as narrow in on single genes. One of these tools is QTL-mapping
. QTL-mapping utilizes a phenomenon known as linkage disequilibrium
by comparing known marker genes with correlated phenotypes. Often, researchers will find a large region of DNA, called a locus
, that accounts for a significant amount of the variation observed in the measured trait. This locus will usually contain a large number of genes that are responsible. A new form of QTL has been described as expression QTL
(eQTL). eQTLs regulate the amount of expressed mRNA, which in turn regulates the amount of protein within the organism.
Another interest of statistical geneticists using QTL mapping is to determine the complexity of the genetic architecture underlying a phenotypic trait. For example, they may be interested in knowing whether a phenotype is shaped by many independent loci, or by a few loci, and do those loci interact. This can provide information on how the phenotype may be evolving.
Epistasis
In genetics, epistasis is the phenomenon where the effects of one gene are modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is called epistatic, while the phenotype altered or suppressed is called hypostatic...
s that together influence a phenotypic
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
trait. The loci or identities of the non-allelic genes are often unknown to biologist. Advances in statistical methodology and high throughput sequencing are, however, allowing researchers to locate candidate genes for the trait. These genes are generally pleiotropic
Pleiotropy
Pleiotropy occurs when one gene influences multiple phenotypic traits. Consequently, a mutation in a pleiotropic gene may have an effect on some or all traits simultaneously...
as well. The genes that contribute to type 2 diabetes are thought to be mostly polygenes.
Inheritance
Polygenic inheritance occurs when one characteristic is controlled by two or more geneGene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s. Often the genes are large in quantity but small in effect. Examples of human polygenic inheritance are height, skin color and weight. Polygenes exist in other organisms, as well. Drosophila
Drosophila
Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...
, for instance, display polygeny with traits such as wing morphology, bristle count and many others.
Trait distribution
The frequency of the phenotypes of these traits generally follows a normal continuous variation distribution pattern. This results from the many possible allelic combinations. When the values are plotted, a bell-shaped curve is obtained. The modeMode (statistics)
In statistics, the mode is the value that occurs most frequently in a data set or a probability distribution. In some fields, notably education, sample data are often called scores, and the sample mode is known as the modal score....
of the distribution represents the optimal, or fittest, phenotype. The more genes are involved, the smoother the estimated curve. However, in this model all genes must code for alleles with additive
Additive
Additive may refer to:* Additive function, a function that preserves the addition operation* Additive inverse, an arithmetic concept* Additive category, a preadditive category with finite biproducts...
effects. This assumption is often unrealistic as many genes display epistasis effects which can have unpredictable affects on the distribution of outcomes, especially when looking at the distribution on a fine scale.
Mapping Polygenes
Quantitative trait locus
Quantitative traits refer to phenotypes that vary in degree and can be attributed to polygenic effects, i.e., product of two or more genes, and their environment. Quantitative trait loci are stretches of DNA containing or linked to the genes that underlie a quantitative trait...
. QTL-mapping utilizes a phenomenon known as linkage disequilibrium
Linkage disequilibrium
In population genetics, linkage disequilibrium is the non-random association of alleles at two or more loci, not necessarily on the same chromosome. It is also referred to as to as gametic phase disequilibrium , or simply gametic disequilibrium...
by comparing known marker genes with correlated phenotypes. Often, researchers will find a large region of DNA, called a locus
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
, that accounts for a significant amount of the variation observed in the measured trait. This locus will usually contain a large number of genes that are responsible. A new form of QTL has been described as expression QTL
Expression quantitative trait loci
Expression quantitative trait loci are genomic loci that regulate expression levels of mRNAs or proteins. Expression traits differ from most other classical complex traits in one important respect—the measured mRNA or protein trait almost always is the product of a single gene with a specific...
(eQTL). eQTLs regulate the amount of expressed mRNA, which in turn regulates the amount of protein within the organism.
Another interest of statistical geneticists using QTL mapping is to determine the complexity of the genetic architecture underlying a phenotypic trait. For example, they may be interested in knowing whether a phenotype is shaped by many independent loci, or by a few loci, and do those loci interact. This can provide information on how the phenotype may be evolving.