Genetics is the science of heredity that involves the study of the structure and function of genes and the methods by which genetic infomation contained in genes is passed from one generation to the next. The modern science of genetics can be traced to the research of Gregor Mendel(1823-1884), who was able to develop a series of laws that described mathematically the way hereditary characteristics pass from parents to offspring. These laws assume that hereditary characteristics are contained in discrete units of genetic material now known as genes.

The story of genetics during the twentieth century is, in one sense, an effort to discover the gene itself. An important breakthrough came in the early 1900s with the work of the American geneticist, Thomas Hunt Morgan (1866-1945). Working with fruit flies, Morgan was able to show that genes are somehow associated with the chromosomes...