150 RADIATION GENETICS 



eight- to ten-hour shift in the diurnal cycle could possibly overcome this problem and 

 would seem worth a try. Since most modern animal quarters are windowless and 

 equipped with resettable automatic light switches, no technical difficulties need be 

 anticipated. 



For the most part, other species either have too few young per cycle or are too 

 expensive to maintain. Nevertheless, a major Atomic Energy Commission-supported 

 program with swine is under way at the Iowa State University, under the direction of 

 Drs. J. L. Lush and L. N. Hazel, which may provide the controlled early mortality 

 data that are required. However, swine after bearing young also tend to trample and 

 crush part of the litter. 



Once the species is selected, the next question concerns the genetic composition of 

 the chosen animals. Should they be inbred, single-cross hybrids, or should they be 

 random-bred or mixed hybrids, such as double crosses? This is not an easy question 

 and the answer any particular investigator gives may not be entirely unbiased. 

 Answering the question raises additional ones on the significance of polymorphism and 

 heterosis in mammalian species. Will inbred animals tend to express heterosis for 

 induced mutation more readily than crossbred animals, or will this expression occur to 

 any significant degree at all? Will genetically heterogenous material have a greater 

 probability of masking the minor detrimental mutations because of an innately greater 

 viability and genetic flexibility ? 



There may be a way out, however, as in the study Green 481 describes, in which 

 four different levels of inbreeding are employed on two different populations — one 

 genetically homozygous and the other genetically heterozygous. An additional 

 procedure is that used by Chapman. 196 In his study, recurrent incrossing and out- 

 crossing is compared with continuous incrossing and outcrossing. In both studies, 

 the animals are irradiated every generation in order to check for cumulative genetic 

 injury but under the different selection pressures induced by the varying degree of 

 inbreeding. The only opinion the writer offers is that the outbred system may be 

 the choice for the sake of its greater comparability to man if the investigators cannot 

 afford to check both inbred and outbred genetic systems. 



RADIATION AS A TOOL FOR GENETIC RESEARCH 



It is an understatement to say that radiation has become an extremely useful tool 

 in experimental biology and medicine. A glance through the contents of this volume 

 should substantiate the fact that many aspects of mammalian genetics have profited as 

 well. No attempt will be made to describe all applications. However, particular 

 contributions to mammalian genetics have been provided through the use of radiation 

 techniques in the areas of physiologic genetics, immunogenetics, developmental 

 genetics, the genetics of disease resistance, the genetics of viability, genetics of cancer, 

 and somatic-cell genetics. Pertinent specific references will be found throughout this 

 volume, and the reader should consult the studies of E. S. Russell, H. Chase, R. D. Owen, 



