Genetics - A. V. Sivolob 2008

Human Genetics
Quantitative and Multifactorial Human Traits

The Patterns of inheritance of qualitative monogenic traits are the most thoroughly studied. However, as mentioned in chapters 3 and 4, genes do not function in "isolation" from other genes and the environment. The expression of a vast number of human traits (as well as those of other organisms) is determined simultaneously by multiple factors: the action of several genes combined with the Influence of Environmental variables (quantitative or multifactorial traits).

The simplest type of inheritance for Quantitative Traits is polygeny, where trait expression depends on the presence of a specific number of dominant alleles of equivalent genes (multiple copies of the same Gene) acting additively within the individual's genotype (Chapter 3). A classic example in humans is Skin pigmentation, which is estimated to be determined by five genes. Nevertheless, for other quantitative traits (such as height, body weight, etc.), the situation is considerably more complex. Although the genes responsible for these traits appear at first glance to act additively (contributing to the continuous distribution of trait variants), they are not equivalent, and various types of interactions may occur among them.

Multifactorial traits can manifest as qualitative ones, where an individual either exhibits the trait or does not. For instance, certain Congenital Malformations (such as anencephaly or Spina bifida) and most common disorders (including myocardial infarction, asthma, atherosclerosis, and "sporadic" cancers) are qualitative traits, yet their inheritance patterns mirror those of quantitative ones. To explain the distribution of such traits within human populations, the threshold model has been proposed: a trait appears only in those individuals in whom the number of "susceptibility genes" exceeds a specific "threshold." In other words, the quantitative trait is the underlying susceptibility to the condition rather than the symptoms themselves. In medical practice, the threshold model is likewise applied to "classical" quantitative traits by categorizing the respective variants into so-called "normal" and "pathological" states—such as dystrophy, normal weight, or clinical obesity. The magnitude of the "threshold" for individual subjects depends on a variety of factors, including sex, lifestyle, and environmental influences. Living conditions can significantly increase or decrease the number of "susceptibility genes" required for a trait to manifest and, consequently, affect the risk of developing the disease.