DEPARTMENT OF GENETICS. 107 



Heredity of Learning Capacity in Mice. 



Miss E. Vicari was a guest of the Department during the summer and 

 continued her investigation into the rate of learning a maze shown by mice 

 of distinct races and hybrids between them, reported upon last year. She 

 has bred the Fi and F2 generations of the hybrids between X-rayed mice and 

 the dilute brown race which have been bred in this Department and which 

 show different reaction times. 



Susceptibility to Inoculable Tumors. 



The experiments on the genetical factors responsible for susceptibility to 

 inoculable tumors, described in earlier Year Books (1921, pp. 122-127; 1922, 

 pp. 113-114), have been carried out by Dr. L. C. Strong, partly in association 

 with this Department, partly with the aid of a grant from the Rockefeller 

 Institute for Medical Research. Last year was reported the discovery of a 

 tumor dbrB, susceptibility to which depended, apparently, upon the presence 

 of at least two independent factors, since a typical 9: 7 ratio was obtained 

 in the F2 generation produced from a cross between a susceptible (dilute^ 

 brown) and a non-susceptible (Bagg albino) strain of mice. Dr. Strong has 

 isolated several genetic families from the F2 generation and continued them 

 by brother-sister matings to the Fe generation. One of these families (known 

 as the E VII) has been selected for high percentage of susceptible individuals. 

 In the F4 generation of this family the following result was obtained : Adding 

 together all of the F4 progeny derived from E VII, F3 matings, in which both 

 susceptible and non-susceptible occurred in the individual matings, the ratio 

 of 94 susceptible to 38 non-susceptible individuals is obtained. This ratio 

 is probably a Mendelian 3: 1 ratio, for an ideal 3: 1 ratio would give 99 

 susceptible to 33 non-susceptible. This result indicates that the F3 parents 

 differed in only one Mendelian factor. Thus Dr. Strong seems to have 

 achieved the long-looked for result of isolating a family in which the differ- 

 ence between susceptible and non-susceptible individuals depends upon only 

 a single gene. The gene has been called the A^*' factor. It was already shown 

 (Year Book 1922, p. 113) that the dbrB strain carried two demonstrable 

 factors, A^* and B^*. The genetic constitution of individuals of the E VII 

 family, as far as their reaction to the dbrB tumor is concerned, is as follows: 

 Non-susceptible individuals have the genetic constitution a^* a^* B^* B^*-|-; 

 susceptible individuals have the genetic constitution A®* a®* B®* B^*-|-, or 

 ^8t ^st gat Bst_|_^ ^Yie difference being the single factor A^^ This family 

 E VII has been continued to the Fe generation. 



Dr. Strong has also made an analysis of the rate of growth of tumors and 

 has demonstrated several important facts which indicate that the difference 

 between two individuals in the growth-rate of the transplanted tumor is 

 caused by genetic differences. Only a part of the evidence may here be cited : 



" (1) Within limits, the growth of the individual tumor is constant. By 

 plotting the logarithms of the observed mass of the tumor at uniform lengths 

 of time after inoculation there is obtained a straight line. When the growth 

 rate is slow, the straight line is evident for 7 or 8 weeks after inoculation, 

 whereas when the growth is rapid, the straight hne is evident for only about 

 4 to 6 weeks (fig. 4). 



"(2) Individuals belonging to the different hybrid generations grow the 

 same dbrB tumor at significantly different rates. For instance, an indi- 



