The inheritance o f hereditary characters in man does not differ in principle from heredity in other organisms, but the study o f human genetics is so important and differs so much from animal or plant genetics in the methods which it employs that it must be given separate consideration. Since controlled breeding is impossible, genetic analysis has to be carried out by special and rather roundabout methods, which will be discussed in this chapter. We shall first consider the study o f rare mutant genes with relatively clear-cut effects and then pass on to the analysis o f the genetic differences which exist within a population o f what we should call “ normal” men and women.