MAMMALIAN RADIATION GENETICS 143 



Cytogenetic analysis. — Two subsequent chapters by Drs. Klein and Yerganian 

 amply cover the methodologies of cytology and cytogenetics. The application of 

 tissue-culture techniques to genetic analysis, radiation and otherwise, is also covered 

 by Drs. Ford and Puck in the previous volume in this series, Methodology in Human 

 Genetics. The use of mammalian cells in tissue culture for radiation-genetic analysis 

 will assuredly be an area of increasing scientific effort. The opportunity has become 

 available to make elaborate comparative quantitative analyses of radiogenetic sensitivi- 

 ties among all important domestic and laboratory animal species. Besides interspecific 

 comparisons, strain differences in radiosensitivity within species can be explored at the 

 cellular level. To date, the recognized genetic differences in radiosensitivity in mice 

 have been attributed to physiologic genetic factors, rather than to basic differences in 

 the resistance or sensitivity to cellular genetic damage. 473, 474, 475 This point can 

 easily be checked in tissue culture. Strains may differ in their sensitivity to mutagens, 

 at least for chromosomal aberration and restitution rates. If so, new techniques 

 would be provided to study the cellular mechanisms of somatic injury and recovery. 



QUANTITATIVE GENETIC EFFECTS OF RADIATION 



Although geneticists have been estimating spontaneous and induced mutation 

 rates in a number of species of plants and animals for many years, virtually all of our 

 present estimates of the potential genetic hazards of radiation to man have been made 

 independently of this accumulated experience. There are several reasons for this. 

 First, while fairly good data for spontaneous mutation rate are available for man, the 

 radiation-induced rate is a complete unknown. Second, the bulk of data on detri- 

 mental genetic factors in man are in the form of morbidity and mortality statistics. 

 Although many clear-cut mutant phenotypes that severely affect viability are recog- 

 nized, their frequencies are generally masked by the normal rates of infant and child- 

 hood mortality in human populations. Thus, more concern is expressed by geneticists 

 about the total mortality than about the individual syndromes of disease con- 

 stituting it. An excellent summary of several hundred detrimental genetic qualities 

 in man has been prepared from experience in Northern Ireland. 1282 Mortality rates 

 at birth and among adults are given, although the temporal patterns of mortality for 

 the various characteristics have not been derived. In addition, the genetic basis for 

 many traits is known to be uncertain and to involve considerable environmental 

 interaction. 



At present, best approximations of the normal load of detrimental genetic factors 

 in man have been derived from the analysis of the progeny of consanguineous marriages. 

 The analytic technique requires the maximum likelihood estimate of the regression of 

 early mortality on the inbreeding coefficient. 901 The procedure has also been em- 

 ployed by Schull 1169 and Slatis 1216 with considerable success. The basic biological 

 data in nearly all instances include some or all of the following : fetal deaths, neonatal 

 and infant deaths, childhood and juvenile deaths. Such data are comparatively easy 



