A Cat gives birth to a kitten, a cow to a calf and a human being to a baby. The members of each of these species resemble one another and possess characteristics common to their respective species. Now the question arises as to what is it that is responsible for a particular type of body, shape and other similar characteristics in the members of one species. On further observation, we find that there are individual differences even in the members of the same species. A child resembles his sisters, brothers, parents, grand-parents, and other members of the family more than the people not related to him at all. What is it that causes such similarities and dissimilarities? The answer usually referred to is heredity which means that an off spring inherits most of the personality traits of his parents and forefathers which make him resemble them. It is in this sense that Douglas and Holland defined heredity in the following words: one's heredity consists of all the structures, physical characteristics, functions or capacities derived from parents, other ancestry species.