256 PHYSIOLOGIC GENETICS 



In view of the fact that we are dealing with a probability that the malignant 

 change will occur and that this probability is increased with increase in time, the 

 temporal factor after the introduction of the carcinogen should be controlled. With 

 susceptible groups, 12 to 16 weeks is about the optimum time for killing the animals, 

 and for most uniform results the mice should be of a uniform age at the time they are 

 injected. 



In order to count the nodules accurately, we routinely inject the lungs endo- 

 tracheally with from 1 to 1.5 ml. of Tellyesniczky's fixative before removing them from 

 the chest cavity. This distends the lungs, causing the tumors to stand out as readily 

 discernible spherical nodules. (This, incidentally, gives an excellent fixation of the 

 tissue for later microscopic study.) The counting is then done with the aid of a dissecting 

 microscope. 



This manner of transforming the problem from that of a threshold character to 

 that of a quantitative character can also be applied to certain other tumors. When 

 papillomas are induced by painting the carcinogens on the skin, multiple papillomas 

 appearand these can be counted as a quantitative measure of response. Such data on 

 papillomas have not been used for heritability studies, but they have been used for 

 mutation studies 197 as will be described later. Induced hepatomas appear as multiple 

 tumors which offer a quantitative measure of response for genetic studies, and even 

 the spontaneous hepatomas are multiple in the highly susceptible strain C3H and in 

 certain hybrids resulting from outcrossing C3H. Data on number of hepatomas have 

 been utilized to advantage in linkage studies. 



Since mammary gland tumors grow rapidly, seldom do more than two or three 

 appear before the female dies, so the number of mammary tumors can hardly be used 

 for a quantitative study. However, as DeOme 251 has clearly shown, mammary 

 tumors are preceded by hyperplastic nodules of the mammary gland which appear 

 in large numbers and can be readily counted. These could be used for a quantitative 

 genetic study. 



Latent period is also a quantitative measure of tumor response and has always 

 been considered along with tumor incidence. Latent period can be most readily 

 determined for such tumors as the mammary tumor that can be observed directly or 

 palpated. In studying pulmonary tumors it is necessary to kill and examine sample 

 groups to determine latent period, but this has been done for a genetic analysis, and the 

 results paralleled those obtained by measuring response on the basis of nodule count. 557 



LINKAGE 



Although cancers are inherited as threshold characters, considerable progress can 

 be made in linkage studies, particularly in the mouse, which now has a rather extensive 

 chromosomal map. (See Dr. Margaret Green's chapter in this volume.) In 1934, 

 Little 798 demonstrated linkage between lethal yellow at the agouti locus and 

 mammary tumors. Since then Bittner 106 has demonstrated linkage of this tumor with 



