Quantitative traits

A quantitative or polygene trait is controlled by two or more different genes working in concert. Eg, hman skin color is controlled by between five and fifteen different genes, each with at least two different alleles. The skin phenotype of an individual is the product of all of the at least 10 alleles of these genes working together, plus the impact of environmental influence. The result of quantitative inheritance is generally that the trait doesn’t show discretely different phenotypes, but rather demonstrates a continuous variation in the species. Human skin color demonstrates this perfectly. There are no “categories” of human skin color, just a continuous spectrum from very pale to very dark.
Many quantitative traits are threshold traits. A threshold trait has basically only two expressions, even though it may be controlled by a number of genes working together. The cumulative effects of all of those genes (and often a significant amount of contribution from environmental factors) determines whether the phenotype will “cross the threshold” from one expression to the other. A probable example of a threshold trait in human beings is schizophrenia. It has been well established that heredity contributes significantly to the possibility that an individual will develop schizophrenia. Each of us inherits a certain level of “predisposition” in our genotypes. Environmental influences can also act to promote the onset of schizophrenia. If one inherits a genotype which contains a high level of genetic predisposition, it can take very little environmental contribution to cross the threshold and trigger the development of schizophrenia. However, if ones genetic predisposition is low, one may be able to accept a high level of environmental influence without developing the symptoms of schizophrenia.
Pliotropy refers to genes with multiple effects. For example, the white spotting gene in gerbils apparently also influences red blood cell count. Another simple example is that genes which influence characteristics of the fingers will also influence characteristics of the toes.