Space does not permit us to tell at length how the facts that are now to be described were discovered. They date from the gardening experiments of an Austrian monk by the name of Mendel, who for many years grew ordinary garden peas and studied from season to season the varieties that appeared. The facts of heredity that he found to be true of peas, have since been shown to apply just as well to ourselves, and since this book deals with human beings we shall try to make the description apply directly to human heredity. The first thing to get clearly in mind is that every single thing about any one of us which can or does differ from the corresponding feature in another person, may be a _hereditary trait_, and, if it is, will have a factor controlling it somewhere among our chromosomes. Since the number of hereditary traits is legion, including not only the size and shape of all parts of our bodies inside and out, but our mental peculiarities and moral tendencies as well, there must be a huge number of controlling factors, or _determiners_, as they are often called. As a matter of fact, there are probably not quite so many determiners as traits, because a single determiner may govern more than one trait. But even so, the number of determiners is too large for comfort in trying to describe their working. The best way to go about it is by pretending that we are not so complicated as we really are. We shall do this by setting the number of different determiners human beings may possess at fifty-two, not because that is anywhere near their real number, but because it is the number of combined large and small letters in the alphabet, and we propose to use letters to stand for determiners, as an easy way to keep them separate in the description.