The study of heredity is called genetics. Genetics is the scientific study of genes and heredity, which involves how certain qualities or traits are passed from parents to offspring as a result of changes in DNA sequence. Heredity is the process by which traits and qualities are passed from parents to their offspring before the offspring are born. Genetics encompasses two seemingly paradoxical observations about organisms: the constancy of a species from generation to generation and the variation among individuals within a species. Constancy and variation are actually two sides of the same coin, as becomes clear in the study of genetics. Genetics as a scientific discipline stemmed from the work of Gregor Mendel in the middle of the 19th century. Mendel suspected that traits were inherited as discrete units, and his units became the basis for the development of the present understanding of heredity.