Quantitative Inheritance

Abstract

Many phenotypes of an organism are not entirely explained simply by Mendelian genetics. Mendelian traits are usually described as discrete variables. Plants are tall or short; seeds are yellow or green; a person is albino or with normal pig-mentation; the eyes of Drosophila are red or some other color. Another mode of inheritance is that responsible for traits displaying a continuous variation; that is, variables that can take any value over a given interval. The magnitude of such traits can be measured and plotted against the frequency of their occurrence in a population. The results will often follow a curve such as that depicted in Figure 6.1. The more common values are close to and on either side of the mean for the population. The rarer values depart more drastically from the mean in either direction. This type of symmetrical distribution is called a curve of normal distribution.