No. 60S] MULTIPLE FACTORS L\ MICK AM) RATS 475 



F2 generation hybrids shows that the conditions on which 

 the growth of the tumor depends are reproduced in hybrids 

 of the two races. The behavior of the Fj hybrid genera- 

 tion produced by reciprocal crosses indicates clearly that 

 even when only one of the parents is susceptible and comes 

 from the homogeneous Japanese race, its contribution to 

 its offspring is sufficiently powerful to produce sus- 

 ceptibility in that animal. In other words, we may say 

 that the hereditary factor or factors underhing suscep- 

 tibility are functional even when present in a "single 

 dose." If there is a single general factor underlying 

 susceptibility we should expect that the Fo generation 

 would show a large number of susceptible animals. This 

 is not the case. It is possible, however, to consider the 

 behavior of the Fj generation as being largely due to 

 heterozygosis and not to true inheritance. To eliminate 

 this possibility a back-cross generation was made between 

 Fi animals and pure Japanese. The sixty-three animals 

 comprising this generation all proved susceptible to the 

 tumor and in a majority of cases grew it as rapidly as did 

 the Fi hybrids themselves. On the other hand, the Fj 

 generation crossed with tame mice gave seventy-eight 

 young, all of which were non-susceptible. 



In discussing the results, it was further suggested that 

 the explanation which best fitted the facts, indeed the only 

 explanation which fits all of the facts, is that susceptibility 

 depends for its manifestation upon the simultaneous pres- 

 ence of several factors in either the homozygous or the 

 heterozygous condition. The gametes of the Japanese 

 race possess all or nearly all of these in a homozygous 

 condition and therefore produce susceptible animals. The 

 Fi hybrids possess all of these factors in a ''single dosf": 

 they Having been contributed by tlic .lapaiu'sc parent, and 

 are therefore susceptible. When, Im/n m-. thr /•', ,in'ninils 

 form gametes, these factors, if theii arc nn'mhUziur/ aud 

 not hJrnd'uifi in nature, will he distrihiitrrl at raurlnm iu 

 the r/amrtrs. Tlio result will be that the lar-er tlie nnni- 

 ber of factors involved, the rarer will be tlie inclusion of 



