Publisher Summary This chapter presents the genetics of human population. The primary point of departure in experimental genetics is to study the hereditary factors that determine certain characters. The chapter discusses three types of characters— namely: (1) hereditary characters in the limited sense; (2) characters due to the environment; and (3) characters depending both on heredity, environment, and constellational. There is also the fact that characters may emerge at different ages. The nature of the population determines to some extent the type under which a character is to be classified. To illustrate this, the chapter discusses a character normally contingent on both heredity and environment– that is, to say a constellational character where the hereditary factors necessary for its occurrence are found only in a part of the population, and where the same is true of the environmental factor or factors that are similarly necessary. In a population where the necessary environmental factor is present in practically all individuals, the character will be determined by heredity; conversely, in a population where the hereditary factor is present in practically everyone the character will be determined by the environment.