Of all the sciences that contribute to the great, tertiary composite which is known as agriculture none is more important economically than genetics. The science which underlies the improvement of plants and animals for agricultural purposes is destined to receive increasing attention in agricultural education and in agricultural practice. Without doubt vast possibilities await realization through the more thorough and systematic development of our living economic resources. Such development is directly dependent upon the successful utilization of genetic principles in plant and animal breeding.  This text has been prepared in response to a real and widely recognized need. The experience of the authors in teaching the principles of breeding to undergraduate students has forced home the conviction that an adequate presentation in a single text of the facts and principles of genetics and their practical applications is a prime necessity.