The science of genetics is concerned with the inheritance of traits, whether normal or abnormal, and with the action of genes in the environment. This latter concept is of particular relevance to medical genetics, since the expression of genes can be modified by manipulating the environment. For example, were we to live in a society that takes in no dairy products, as well as avoids breast-feeding, we would not be aware of the inherited disease, galactosemia. Human genetics can be subdivided into population genetics, cytogenetics, biochemical genetics, immunogenetics, somatic cell genetics and, of course, applied or clinical genetics. While there is a great deal of overlap among these subdivisions, I shall deal with each separately, both for the definition of terms and a review of their principles and applications.
Population or mathetmatical genetics deals with the study of the mode of inheritance of traits and with the distribution