The traits that we all express are the result of both the genes which we inherit from our parents, and the environment in which we live. Some traits have a purely genetic component, and follow the rules of Mendelian inheritance, whereas others show far more complex pedigree patterns. ... Due to the considerable complexity of many traits, the exact genetic and environmental contributions are not known or understood. However, with recent technological advances in biotechnology, the prospects for understanding such complex traits are improving. DNA sequencing and advances in the use of markers are enabling rapid progress in the understanding of such traits. This is particularly significant in medicine, as diseases are often complex traits and understanding of the factors affecting their expression will be of great significance in finding treatments for, or preventing such diseases from being expressed.
A polygenic trait is a trait for which the phenotype depends on alleles at many different genes. ... Polygenic traits often show a difference in degree among related individuals, and the greater the number of genes that affect the phenotype, the more continuous the distribution becomes. ...
Genetic variation is increased by the polygenic traits. ... Such traits are called threshold traits. With such traits, a particular phenotype is only expressed once the additive effect of the relevant genes accumulates to a threshold level. ... In order to study evolution, it is therefore necessary that we can estimate the amount of genetic variation in a trait. For traits coded for by a single gene, it is possible to evaluate allele frequencies and therefore genotype frequencies by use of the Hardy-Weinberg Model. This is not possible for polygenic traits as there are too many genes affecting each trait. ... However, the advances of genome projects in many species, and the increasing numbers of polymorphisms that have been characterised and mapped has made the task of identifying polygenic trait loci more feasible. ... The affected sib-pair method can also be used to test for increased marker similarity in affected sib pairs, and the affected-pedigree-member method can be used in the same way, but comparing affected individuals using extended families instead of siblings.
When highly inbred lines continue to show variation in a polygenic trait, the variation is attributed to environmental causes rather than genetic causes. ... Traits that have a complex cause involving both environmental and genetic components from many different, possibly interacting loci are known as complex traits. An example or a complex trait is height, where height is partly dependant on the genes inherited from an individuals parents, but also partly dependant on the quality of nutrition that the individual received as a child.
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