Every cell of the human body contains a set of biological blueprints that enable it to perform its essential functions. This information is contained in chromosomes, which are composed of DNA. Genes are segments of DNA that serve as the basic unit of our heredity. Our genes assembled in complex combinations and along with environmental factors determine significant aspects of our biological make up. Genes do not control behaviour or other aspects of life directly; rather, they do so indirectly by impacting on chemical reactions in the brain and other parts of the body. Heredity is of two types, biological and social. Biological heredity is the one that the child inherits from his parents and forefathers in the form of chromosomes. Social heredity includes all that one generation inherits from preceding generations in the form of social traditions, customs and skills etc. Heredity and environment both play an important role in the development of personality. Human personality comes into shape by the interaction of heredity and environment. Heredity influences the individual physique, motor sensory equipment and level of intelligence. It also influences temperamental characteristics and proneness to diseases.