MAMMALIAN RADIATION GENETICS 135 



genetic behavior of the trait, but cytogenetic proof has also been obtained. 39, 731 

 Embryonic deaths appear to occur largely at or soon after implantation. 973 



Partial sterility resulting from induced reciprocal translocation is almost entirely 

 restricted to the mature-germ-cell stages, possibly only significantly in spermatozoa. 

 It has been suggested that aberrations of this type would not survive meiosis and there- 

 fore the yield in gonial cells would be negligible. This has been challenged by 

 Griffen 497 but his data await complete cytologic confirmation. The simple use of 

 reduced litter size as the detector can be misleading, although it is the normal procedure 

 of first screening for semisterile animals. The frequency in offspring from irradiated 

 females is much less than in offspring from irradiated males. 1127 Since the germ cells 

 of the mature ovary are largely primary oocytes, this finding indirectly implies that the 

 most mature germ cell is more sensitive to the induction of a transmissible partial 

 sterility ; germ-cell death probably culls out this damage in the less mature cells. 



The dose-response data are erratic. 1128 Theoretically, one would expect the yield 

 of semisterile mice to be linearly related to the square of the dose since a two-hit 

 aberration is involved. The early work, summarized in the above reference, does not 

 appear to demonstrate any clear-cut dose-response relationship. The data obtained 

 by Charles et al. 198 do, however, fit a (dose) 2 function with a greater reduction of the 

 variance than the simple linear-arithmetic function employed by the authors. In the 

 experiment of Charles, male mice were exposed to four different daily dose levels of 

 X rays but at a constant dose rate. Thus, the physical factors were appropriate for 

 the use of the D 2 function. Matings were continually carried out so that the total 

 dose levels were average values and not carefully separated points. This, along with 

 the fact that there was no sure way of knowing what irradiated cell stage produced the 

 trait, detracts from any quantitative test."}" 



Dominant lethals. — The induction of dominant lethals in male germ cells can be 

 detected by measuring the reduction in litter size produced by the irradiated sire, 

 although litter size at birth is not usually considered the most accurate measure. A 

 careful evaluation requires sacrifice of the pregnant female at about 15-17 days post- 

 conception and the counting of (a) number of live embryos, (b) number of dead 

 embryos, (c) number of corpora lutea, and (d) number of pre-implantation deaths. 

 Most of the losses occur prior to 10-11 days of gestation and pre-implantation losses are 

 closely correlated with dose. 1141 Post-implantation losses rise to about a 20-25 per 

 cent representation of the number of corpora lutea at doses below 100 r and remains at 

 that level. Cytologic studies have demonstrated that the lethal action is pre- 

 dominantly due to aberrant cleavage and chromosomal fragmentation. 128 



f The Charles experiment, carried out at the University of Rochester, was nevertheless 

 the first major concerted effort to evaluate the genetic hazards of radiation in a mammal 

 and was set up under the auspices of the Manhattan Engineering District, predecessor of the 

 Atomic Energy Commission. Preparation of a final report was delayed by the untimely 

 death of Dr. Charles, but the report is now being published as a University of Rochester- 

 Atomic Energy Commission Project Report No. UR-565 and should soon become available 

 to interested geneticists. 



