One of the most remarkable cases of heredity in man is found in the so-called blood groups. As first definitely shown by Von Dungern and Hirschfeld in 1910, the inheritance of the four blood groups conforms to Mendel’s laws. So consistent is this relation that, as Ottenberg pointed out in 1921, the evidence might be used in certain cases to determine the parentage of the child. Since this statement has recently been disputed by Buchanan, from an entirely wrong interpretation of Mendel’s principles, I should like to point out that on the Mendelian assumption of two pairs of factors, all the known results are fully accounted for. If we represent one pair of genes by A and a and the other pair by B and b, and if we represent an individual with the genetic constitution AaBb mating with another individual of like constitution (AaBb), then each will contain four kinds of germ cells, viz., AB, Ab, Ba, and ab. The sixteen possible combinations formed if any sperm may fertilize any egg are shown in Fig. 8. These sixteen individuals fall into four groups according to whether they have both A and B, or only A, or only B, or neither A nor B (_i.e._, ab) in the proportion of 9AB:3A:3B:1ab. These four genetic classes correspond to the four recognized blood types IV, II, III, I, as indicated in the diagram. Now these sixteen kinds of individuals are found in all populations, so far studied, although in somewhat different proportions in different “races.” It is very simple to tell what the kinds of genetic offspring will be where any one of these sixteen individuals marries any other one. These possibilities are summarized in the following statement taken from Ottenberg: Unions of I and I give I I II } } I, II II II } I III } } I, III III III } Unions of II and III give I, II, III, IV. IV I I, II, III, IV. IV II I, II, III, IV. IV III I, II, III, IV. IV IV I, II, III, IV. Two actual pedigrees, one of them carried through three generations, will serve to illustrate particular cases, Fig. 9. From a knowledge of the blood group to which the child belongs it is possible to predict to what groups its parents may have belonged, and in certain cases it is possible to state that an individual of a certain group could not have been the parent of a particular child. [Illustration: FIG. 9. The upper pedigree gives the children from the family in which types I and IV were the parents. The offspring belong to types II and III (two of the four possible kinds of offspring). The lower pedigree represents three generations. The grandparents are I and II and I and III, respectively, while the parents are II and III.]