SOME POSSIBLE BEARINGS OF GENETICS ON PATHOLOGY 17 



and b, and if we represent an individual with the genetic consti- 

 tution 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 : 3 A : 3B : iab. 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: 



I. II 



I, III 



Unions of II and III give I, II, III, IV. 



IV I I, II, III, IV. 



IV II I, II, HI, IV. 



IV III I, II, III, IV. 



IV IV I, II, III, IV. 



Two actual pedigrees, one of them carried through three gen- 

 erations, will serve to illustrate particular cases, Fig. 9. 



From a knowledge of the blood group to which the child be- 

 longs it is possible to predict to what groups its parents may have 

 belonged, and in certain cases it is possible to state that an indi- 

 vidual of a certain group could not have been the parent of a par- 

 ticular child. 



