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When multiple (poly) genes unite to result in a single phenotype, this is called “polygenic inheritance.” Examples of traits that are produced by multiple genes are height, shape, weight, color and metabolic rate. Polygenic traits are often recognized by their continuous variation, or small differences. To illustrate, height in humans is a polygenic trait. In one class of schoolchildren, there will be an average height among the peers and then a range of variations: those who are very tall and those who are very short. These variations of traits are usually caused by multiple gene pairs. This is an example of polygenic inheritance.
Polygenic traits are distinguished by three criteria: Polygenic (quantitative) traits can be measured in some way (height, sizes of different parts of the body, degree of some aspect of color); two or more gene pairs contribute to the phenotype; and these traits vary across a wide range.
Aside from the physical characteristics mentioned above, other human polygenic traits include SLE (lupus), intelligence and many forms of behavior.
List of Polygenic Traits
Polygenic traits, governed by environmental factors are
Polygenic traits examples with disorders in genetic components are
Congenital heart disease
Congenital dislocation of hip
Neural tube defects
Ischaemic heart disease
Pleiotropy, on the other hand, is the effect of a single gene on more than one characteristic. The term pleiotropy is derived from the Greek words pleio, which means “many,” and tropic, which means “affecting.” In 1936, scientists Landauer and Upham observed that chickens with the dominant frizzle gene developed feathers that curled outward rather than lying flat against their bodies as in standard chickens. Aside from producing faulty feathers, the frizzle gene caused other phenotypic effects, including higher metabolic and blood flow rates, abnormal body temperatures and greater digestive capacity by the chickens. Also, chickens that had this dominant frizzle gene produced fewer eggs than the standard chicken and displayed changes in their heart, kidneys and spleen.
Pleiotropic genes can also be found in humans, and are often linked with congenital diseases. When a mutation in one gene causes a variety of negative symptoms for an individual, this is an example of pleiotropy at work. Sickle cell anemia is a form of pleiotropy, as it is caused by a unique mutation in one gene that leads to a wide range of symptoms. Another example of pleiotropy in humans is phenylketonuria (PKU). This disease is caused by a deficiency of the enzyme phenylalanine hydroxylase, which is necessary to convert essential amino acids in the body. A flaw in the single gene that corresponds to this enzyme causes many symptoms associated with PKU, including mental retardation, scaly skin and pigment defects that make sufferers appear lighter skinned.
Although researchers are inclined to study pleiotropy in order to better understand mutations, in fact, pleiotropy can also occur in perfectly normal genes as well.
On a positive note, the multiple effects that a single gene can cause offers scientists valuable insight into the specific biological function of each and every gene. Pleiotropy can also provide important information about the makeup and development of individual genes over time. This is especially significant in this day and age when genes are being resculpted for new purposes outside of their original roles. In other words, pleiotropy has brought to light that most genes have multiple roles in determining the expression of human traits, and therefore, any genetic alteration can potentially effect a variety of human tissues in myriad ways.
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