Description of Quantitative Traits
Genetic and Environmental Effects
As you know, if the allelic interactions known for a particular gene the genotype can be used to predict the phenotype. With one gene controlling a trait we have three possible genotypes, AA, Aa and aa and depending on the allelic interactions (dominance or incomplete dominance) we can have two or three phenotypes. As more and more genes control a trait, a greater number of genotypes are possible. The formula that predicts the number of genotypes from the number of genes is 3 to the power n. (n is the number of genes.) The following is the number of genotypes for a selected number (n) of genes which control an arbitrary trait.
Let's look at an example with two genes, A and B. We will assign metric values to each of the alleles. The A allele will give 4 units while the a allele will provide 2 units. At the other locus, the B allele will contribute 2 units while the b allele will provide 1 units. With two genes controlling a trait, nine different genotypes are poss ible. Below are the genotypes and their associated metric values:
These results can be presented in a grapical format.
The above graph shows the distribution of the data in the above table. This graph has the bell-shaped curve that is indicative of the normal distribution. This has important implications for the manner in which quantitative traits are analyzed.
This example demonstrates additive gene action. This means that each allele has a speicific value that it contributes to the final phenotype. Therefore, each genotypes has a slightly different metric or quantitative value that results in a distribution (or curve) of metric values that is similar approach a continuous curve.
Other genetic interactions such as dominance or epistasis also affect the phenotype. For example, if dominant gene action controls a trait, than the homozygous dominant and heterozygote will have the same phenotypic value. Therefore, the number of phenotypes is less than for additive gene action. Furthermore, the number of phenotypes that result from a specific genotype will be reduced further if epistatic interactions between several loci affects the phenotype. Additive, dominance, and epistatic effects can all contribute to the phenotype of a quantitative trait, but generally additive interactions are the most important.
All of the above factors are genetic in nature, but the environment also affects quantitative traits. The primary affect of the environment is to change the value for a particular genotype. Using our example above, the value for the genotype AaBb might vary from 8-10. This variation would be the result of the different environments in which the genotype was grown. The consequence of this environmental effect is that the distribution even more resembles a normal distribution.
To illustrate the effect of environment on the expression of a genotype, look at the yields of winter wheat at one North Dakota location (Casselton, ND) during the last ten years. (The data was kindly provided by Dr. Jim Anderson, Dept of Plant Sciences, North Dakota State University, Fargo, ND.) Any year for year variation in yield for any one genotype is largely an effect of the environment.
Note: All plants in 1990 experienced winter kill.
Therefore, the phenotype is a sum of the environmental and the genetic effects. Stated in a mathematical format:
Phenotype = Genetic Factors + Environmental Factors
And one of the goals of quantitative genetics is to measure the contribution of genetic and environmental factors on a specific phenotype. As you might imagine,the field of quantiative genetics also studies other aspects of quantitative traits.
Questions Studied By Quantitative Geneticists