146 RADIATION GENETICS 



inbred strains and hybrids. These are: A/He, A/ Tax, BALB/c, C3H f /He, C57BL/6, 

 and Fi and F 2 hybrids from the BALB/c x C57BL/6 cross. Rather large numbers of 

 mouse days of experience are required for adequate analysis of the young adulthood 

 portion of the life table. For example, the BALB/c x C57BL/6 F x hybrid at weaning 

 age has a death rate approaching a low of 1 in 10,000 per day. Prior to this time and 

 beyond 150 to 200 days of age for most mice, the daily rates of death are high enough 

 to permit reasonable data to be derived from samples of only 100 to 200 mice when 

 50- to 100-day intervals are used. The greatest advantage of using life tables lies in 

 their more direct comparability to human experience. The full form of the life table 

 is remarkably similar for most mammalian species. This similarity has been employed 

 to extrapolate the life-shortening effects of radiation from mice to man. 333 - 1144 All 

 analyses to date have concerned only adult populations, however. 



Figure 19 shows a comparison of murine and human populations in the neonatal, 

 infant, and childhood periods. The U.S. population data were obtained from the 

 published vital statistics of the United States available from the National Office of 

 Vital Statistics, U.S. Public Health Service. The data for mice are some of those of 

 the writer's. The F 2 was derived from the reciprocal crosses of the BALB/c and 

 C57BL/6 inbred strains, and the referred inbred in figure 19 is the BALB/c parental 

 line. Although these data are for the combined sexes, routinely sex is determined at 

 birth and litters are checked daily for deaths. For the sake of ease in final recording 

 and coding of IBM punch cards, the data are assembled in the intervals : 0-5 days, 

 6-15 days, and 16 days to weaning, which provides a mean of approximately 30 days 

 of age. In time, the data will be analyzed by sex and strain for maternal age, parity, 

 and effects of litter size. Preliminary analysis of the BALB/c strain, for example, 

 shows that litters of 1 to 3 mice have a 2- to 3-fold greater mortality rate during the full 

 preweaning period than do litters of 4 or more. 



The point of interest in figure 19 is the nearly complete superposition of the human 

 and mouse infant mortality data. The ratio of time scales is approximately 120:1. 

 This ratio is 2 to 4 times greater than that customarily noted in the comparison of adult 

 populations and undoubtedly reflects the rather attenuated prepubertal develop- 

 mental period of man. Physiologically, the first 30 days of life for the mouse and the 

 first 10 years of life for man are generally comparable. For example, it is recognized 

 that a small proportion of the females of both species will reach sexual maturity by 

 the end of the indicated age intervals. Many of the major hereditary defects in 

 man express themselves during this early period of life. The same is undoubtedly 

 true for mice and other mammals. The basic similarity in the temporal course of 

 spontaneous mortality for the two species certainly encourages more complete and 

 quantitative study of laboratory animals in order to evaluate the general adherence of 

 the expression of genes for viability to genetic theory and expectation. Of particular 

 note here are the data of Russell and Russell 1138 indicating that most of the recessive 

 lethals detected in the specific-locus tests induce death in the neonatal and preweaning 

 period rather than in prenatal life. 



