Polygenic Inheritance and Disorders
The inheritance of polygenic traits depends upon the interaction of two or more genes. This is sometimes confused with the idea of multiple alleles, which are just different forms of the same gene. However, the two concepts are related when discussing the probable phenotype resulting from a particular genotype. Such diseases as diabetes mellitus or heart disease are not the consequence of single gene inheritance, likewise physical traits such as height, weight or even behavior are all examples of quantitative traits whose expression depends upon several different factors. These include the number of genes involved, the number of alleles each gene has, and how much the phenotypic variability depends upon environmental interactions. Unlike qualitative traits such as blood type or other multiple allelic genes that show an unambiguous phenotype, quantitative polygenic traits show a range expression. The distribution of height, for instance, is expressed as short at one extreme and tall at the other extreme, with the bulk falling somewhere in the middle. Height can be used again when discussing the role environment plays in the expression of polygenic traits. Consider the change between the first generation of Japanese-Americans, to that of the second generation who ate an American diet.
Another example would be the height of a tree grown in a temperate climate compared to the same one grown under drought conditions. The genotype would be the same, but the phenotype would be different due to lack of water.
Many different diseases show polygenic inheritance patterns. Hip dysplasia in dogs illustrates this expression well. Imagine two dogs with healthy hips are mated and produce offspring with the result that some of the puppies are completely crippled, some of them seem normal, but x-rays show they have hip dysplasia, and some of the puppies are not affected. This gradation from severely dysplastic to normal is the result of a cumulation of mutations that cause the disease to finally reach a threshold level and to then be expressed. Each of the parents has some of the mutations that cause the disease, but not enough to express the disease in themselves. However, when the two are bred, some of the their puppies get enough of the "bad" genes to have the severe form of the disease, some of the puppies get enough to show the moderate form of the disease and others were lucky and did not get enough of the deleterious genes to be affected, but are most likely carriers of some the disease genes as their parents.
Twin studies in humans have been useful in showing how the expression of polygenic traits is influenced by the environment. For example, two twins may have the potential to become diabetic, but because of their different diets, one becomes diabetic and the other does not. Because environmental factors can play such a large part in the expression of diseases that are polygenic, many people are not aware they carry these genes until their offspring accumulate enough of the defective genes to express the disease. This makes these types of disease hard to eradicate.
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