Sec. 3.1 THE DETERMINATION OF PROBABILITIES 57 



Each person is considered to have in the cells of his body twenty- 

 four pairs of chromosomes, one member of each pair having come 

 from each parent. On these chromosomes are carried genes which 

 are believed to govern the inheritance of various human character- 

 istics. The genes which determine the presence or the absence of the 

 A and B factors in the blood are carried on one of the twenty-four 

 pairs of chromosomes. Attention here is centered solely on the chro- 

 mosomes of that pair, one of which came from the father, the other 

 from the mother. Moreover, attention is to be fixed upon one spe- 

 cific gene position on each such chromosome, namely, that position 

 occupied by the genes which cause the presence or the absence of the 

 A and B factors. If the gene at this position produces neither the 

 A nor the B factor in the blood, it is marked (diagrammatically) 

 as an gene. Similarly there are A and B genes so that the blood 

 type can be indicated by showing what genes the two chromosomes 

 carry. Symbolically, there are the following four blood types: 



O/O = type 0; A/0 or A/A = type A; B/0 or B/B = type B; and 



A/B = type AB. 



The information presented in the preceding paragraphs makes it 



possible to predict the proportions of the various blood groups among 



the progeny of any particular combination of parents, provided that 



a large number of such parents and children are involved. Suppose 



that one parent has blood of type AB and the other has type blood. 



Then the possible blood types which can occur among their offspring 



are as follows: 



Father Mother 



Parents A/B X 0/0 



Genes passed on A or B or 



"B/0 



Possible offspring 



B/0 

 A/0 



[A/0 



Of the four possible pairings of chromosomes from the father and 

 the mother, two produce type B blood in the child because only the 

 gene for the factor B is carried on the chromosomes. Similarly, the 

 other two possible pairings of chromosomes produce type A blood in 

 the children. There is no reason to doubt the usual hypothesis that 

 each of these four possible pairings occurs the same percentage of the 



