GENETICS is the study of heritable variation. The ultimate form the organism takes, its phenotype, is the result of environmental influences imposed upon the potentialities determined by the inherited genetic material, the genotype. Since the inherited capacities are fixed at the time of fertilization and in general do not change throughout the life of the organism, while the environment is ever variable, the study of genetics cannot help but offer an important source of information concerning biological mechanisms. The impact of genetics upon medicine has not been very great in the past, but an increasing interest in the metabolic aspects of disease has led to a recognition of the genetic control of such processes, and it is probable that in the future this subject will become increasingly important.
The civilization of man has been exemplified by an increasing concern with the preservation of the life and well-being of individuals, and physicians have emerged as an agency for the counteraction of the action of natural selection. Natural selection may be visualized as limiting extravagant variation, while conserving the latent capacity to vary according to changing environmental requirements. In medicine we seek ways to alter the environment for particular individuals so as to increase and preserve their ability to survive and to reproduce, thereby maintaining variants which might be otherwise eliminated.
The contribution to biological variability of the genetic material has been the subject of much study, and it has been pointed out that the favorable or unfavorable selective value of particular genes may best be expressed in terms of their influence upon the ability of the individual to reproduce.