The sum of heredity, of all biological processes by which characteristics are particular can be transferred from parents to their children. The theory of heredity encloses two apparently paradoxical observations about organisms: the reliability of a species from one generation to generation and the difference among individuals within a species. The actually two sides of the coin having the same, has becomes plain in the study of genetics. Both aspects of inborn traits can be explained by genes, the functional units of DNA and RNA material that are found within all living cells.