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Quantitative trait locus
Quantative trait locus (QTL) is a region of DNA that is associated with a particular trait (e.g., plant height). Though not necessarily genes themselves, QTLs are stretches of DNA that are closely linked to the genes that underlie the trait in question.
Typically, QTLs underlie continuous traits (those traits that vary continously - the trait could have any value within a range - e.g., height) as opposed to discrete traits (traits that have two or several character values - e.g., number of leaves on a plant or smooth vs. wrinkled peas used by Mendel's in his experiments).
Moreover, a single phenotypic trait is usually determined by many genes (a phenomenon called epistasis). Consequently, many QTLs are associated with a single trait - these QTLs can even be found on different chromosomes. Knowing the number of QTLs that explains variation in the phenotypic trait tells us about the genetic architecture of a trait. It may tell us that plant height is controlled by many genes of small effect, or by a few genes of large effect.
Another use of QTLs is to identify candidate genes (Quantitative Trait Genes , QTG) underlying a trait. Once a region of DNA is identified as contributing to a phenotype, it can be sequenced. The DNA sequence of any genes in this region can then be compared to a database of DNA for genes whose function is already known. In this way, we might find a gene involved in the production of growth hormone.
In a recent development, classical QTL analyses are combined with gene expression profiling i.e. by DNA microarrays. Such expression QTLs (e-QTLs) describe cis- and trans-controlling elements for the expression of often disease-associated genes. Observed epistatic effects have been found beneficial to identify the QTG by a cross-validation of genes within the interacting loci with metabolic pathway- and scientific literature databases.
QTL mapping is the statistical study of the alleles which occur in a locus and the phenotypes (physical forms or traits) that they produce. Because most traits of interest are governed by more than one gene, defining and studying the entire locus of genes related to a trait gives hope of understanding what effect the genotype of an individual might have in the real world.
Statistical analysis is required to demonstrate that different genes interact with one another and to determine whether they produce a significant effect on the phenotype. QTLs identify a particular region of the genome as containing a gene that is associated with the trait being assayed or measured. They are shown as intervals across a chromosome, where the probability of association is plotted for each marker used in the mapping experiment.
The QTL techniques were developed in the late 1980s and can be performed on inbred strains of any species.
To begin, a set of genetic markers must be developed for the species in question. A marker is an identifiable region of variable DNA.
Biologist are interested in understanding the genetic basis of phenotypes (what an organism looks like). Ideally, they would be able to find the specific gene or genes in question, but this is a long and difficult undertaking. Instead, they can more readily find regions of DNA that are very close to the genes in question. When a QTL is found, it is often not the actual gene underlying the phenotypic trait, but rather a region of DNA that is closely linked with the gene.
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