GENETICS OF NEOPLASIA 253 



were functioning. This technique is of value in studying the path of action of the lethal 

 yellow gene in the etiology of neoplasms as will be discussed later. 



Recently, Krohn 734 succeeded in transferring ovaries homologously . He obtained 

 young from both C3H and CBA ovaries transplanted into strain A females rendered 

 tolerant by injection of splenic cells from the donor strains. Such techniques could not 

 be used for developing agent-free substrain because the virus would be transferred by 

 the splenic cells and the ovarian tissue, but it could conceivably be used for certain 

 studies on neoplasia in which one wanted to separate genetic influences from possible 

 intrauterine influences. 



USE OF THE F x HYBRID 



Except for variation between males of reciprocal crosses, F ± hybrids between two 

 highly inbred strains should be as uniform genetically as the parental strains. In 

 certain cases, owing to the position of the genetic level in respect to the threshold, the F 1 

 hybrid may show even greater phenotypic uniformity. For example, one of the parental 

 strains may have an incidence of a certain neoplasm approximating 50 per cent, or 

 maximum variability, and the incidence in the F x hybrid resulting from outcrossing to 

 a very resistant strain may be less than 5 per cent, or minimum variability. This 

 genetic uniformity together with the hybrid vigor contributing toward long-lived, 

 healthy animals makes F x hybrids especially suitable for test animals. Furthermore, 

 special types of F x hybrids can be produced for specific tests. For example, much of 

 the testing for the mammary-tumor agent has been done in F x females from the cross 

 between low-tumor-strain females and high-tumor-strain males. These hybrids are 

 genetically susceptible to the agent although they do not have it except in rare instances 

 when the agent has been transmitted by the male. 



The value of reciprocal F 1 hybrids in revealing extrachromosomal factors in the 

 etiology of cancer is well known. One of the best known cancer viruses, the mammary- 

 tumor virus, was discovered by geneticists as an extrachromosomal maternal influence 

 revealed through the genetic procedures of making reciprocal crosses between strains 

 having high and low incidences of mammary cancer. 1266 In contrast, reciprocal 

 crosses in genetic studies of leukemia have revealed an extrachromosomal resistance 

 influence contributed by the low leukemic mother. 763, 837 



SPECIES OTHER THAN THE MOUSE 



The value of mammalian species other than the mouse should not be overlooked 

 for genetic studies of certain neoplasms. The sarcomas of the liver and subcutaneous 

 tissue of the rat induced with Cysticercus fasciolaris offer a unique opportunity for a 

 genetic analysis of host-parasite relationship in the etiology of cancer. 309 This is the 

 only parasite larger than a virus that is now known to cause cancer under laboratory 

 conditions. 



