374 THE AMERICAN NATURALIST [Vol.LIV 



This shows how the immunity to this staphylococcus disease of 

 the large albino strain as opposed to the predisposition to it of 

 the Japanese strain, is completely dominant in the hybrids. 



To our great surprise we found that this difference between 

 immunity and predisposition was caused by presence or absence 

 of one single genetic factor. In other words, we found a very 

 clear monofactorial Mendelian segregation in F 2 . As we are 

 weighing non-breeding F., animals up to a relatively high age, 

 thirty-one litters containing 125 animals fell into this class be- 

 tween the two dates. 



Of these 125 animals 91 were living on February 14, and 34 

 had died. (Theoretical expectation 93.75:31.25.) See Table I. 



If in reality the "Robertson" mice have one gene, lacking in 

 the Japanese, whose presence protects them against death from 

 this int'eetion, we would expect the hybrids to produce 50 per 

 cent, gametes with and as many without this gene. As the 

 Japanese lack this gene, w T e would expect 50 per cent, of the 

 young from matings between F x and Japanese to be immune, 

 and 50 per cent, to die. Fourteen such litters were available 

 for the test, with 57 animals. Of these 57 on February 14, 

 there were 25 left, 32 having died. (Theoretical expectation 

 equality.) See Table II. 



TABLE II 



In the same class with the other litters we had sixteen litters 

 of young, each from one F a and one "Robertson" parent. This 

 gave us 51 mice in this class. Fifty of these were living Feb- 

 ruary 14, one having died. (Theoretical expectation no deaths.) 



As will be seen in nearly every case the number of deaths 

 was slightly greater than expectation. Occasional mice will die 

 even when given the best of care. It is indeed remarkable that 

 not more of these vigorous mice, kept for the most part in com- 

 pany with several of their own sex, got killed fighting. Tt must 



