THE GENETICS OF TUMOR TRANSPLANTATION 281 



and it gave somewhat striking and startling results. It depended upon the 

 hypothesis that certain characters of an organism depended upon the 

 simultaneous presence of more than one Mendelizing gene. 



Certain characters of this type were already known. Since it was this 

 hypothesis which was applied successfully to the reaction of mice to trans- 

 planted tumors it may be discussed to advantage at this time. 



F| 



PARENT I. 



PARENT 2. 



SUSCEPTIBILITY 



\ 



NON-SUSCEPTIBILITY 



SUSCEPTIBILITY GENE PRESENT 



SUSCEPTIBLE 



I 

 NON-SUSCEPTIBLE 



PERCENT SUSCEPTIBLE 75.0 



Fig. 127. — Diagram showing the inheritance of susceptibility to transplanted tumors 



where susceptibility is due to the presence of a single dominant gene. 



Characters dependent upon one pair of genes. — It is, of course, well 

 known that Mendelian inheritance when one pair of genes is involved gives a 

 3 : 1 ratio in F2, in this case 3 susceptible mice to i non-susceptible mouse 

 (Fig. 127). 



Characters dependent upon two pairs of genes. — If now we suppose that 

 two genes, A and B, are needed simultaneously to produce susceptibility we 

 should have a ratio, not of 3:1, but of 9:7 or 1.3:1 (Fig. 128). 



Characters dependent upon three pairs of genes. — If we continue this 

 principle to a character dependent upon the simultaneous presence of 3 

 genes, the ratio will change still further (Fig. 129). 



