Quantitative Traits

An important goal of genetic studies is to characterize the genetic architecture of quantitative traits. Genetic architecture can been defined in one of four ways. First, it refers to the number of QTLs that influence a quantitative trait. Second, it can mean the number of alleles that each QTL has. Third, it reflects the frequencies of the alleles in the population. And fourth, it refers to the influence of each QTL and its alleles on the quantitative trait. Imagine, for instance, a quantitative trait influenced by 6 loci, each of which has 3 alleles. This gives a total of 18 possible allele combinations. Some alleles may be very rare in a population, so that the phenotypes it contributes to may be rare as well. Some alleles have disproportionate effects on the phenotype (for instance, an allele that causes dwarfism), which may mask the more subtle effects of other alleles. The trait may also be influenced by the environment, giving an even wider range of phenotypic possibilities.

Understanding the genetic architecture of quantitative traits is important in a number of disciplines, including animal and plant breeding, medicine, and evolution. For example, a quantitative trait of interest to animal breeders might be meat quality in pigs. The identification and characterization of QTLs for meat quality might provide a basis for selecting and breeding pigs with certain desirable features. In medicine, an important goal is to identify genetic risk factors for various common diseases. Many genetic studies of common disease focus on the presence or absence of disease as the trait of interest. In some cases, however, quantitative traits may provide more information for identifying genes than qualitative traits. For example, Human height is a quantitative trait, controlled by multiple genes. The broad distribution of heights reflects this fact. Note that most people have an intermediate height, a typical distribution pattern for quantitative traits. identifying genetic risk factors for cardiovascular disease might be facilitated by studying the genetic architecture of cholesterol metabolism or blood pressure rather that the presence or absence of cardiovascular disease itself. Cholesterol metabolism is an example of an intermediate trait or endophenotype for cardiovascular disease. That is, it is related to the disease and may be useful as a "proxy measure" of the disease.