MAMMALIAN RADIATION GENETICS 131 



tion of these factors points out a number of interdependent relationships which can at 

 times result in difficulties when attempts are made to isolate the effect of some one 

 particular factor. 



Variations in the pattern of exposure have been employed for years by geneticists 

 to assist in understanding the kinetics of chromosomal breakage and restitution. The 

 simple use of paired doses usually has been the method of choice. The effects of a 

 given total dose, delivered in a single exposure for determination of base-line, is com- 

 pared to the effects following the same total dose delivered in two equal parts with a 

 varying interval between halves. The value of this procedure for cytogenetic studies, 

 which have largely been done with plant material and Drosophila, needs no documenta- 

 tion here, but apparently the paired-dose method has not been employed for 

 mammalian genetic studies. Weekly and daily fractionations of dose have been used, 

 however, in a number of studies on the induction of sterility. 328, 1085, 1087, 1114 Rather 

 than extend the discussion of exposure pattern, reference will be made to certain 

 applications of these variables at pertinent places in the discussion of genetic tests. 

 The following summary of exposure variables is therefore given. 



I. Single exposure; with variation in: 



a. Total dose 



b. Dose rate 



II. Fractionated exposure; with variation in: 



a. Total dose 



b. Size of individual dose or fraction 



c. Number of individual fractions 



d. Dose rate/fraction 



, e. Interval between fractions 

 f. Protraction period or interval between first and last fractions 

 III. Continuous exposure; with variation in: 



a. Total dose 



b. Dose rate 



c. Protraction period 



Of course, these factors are fully applicable to the use of external radiations. 

 When internal emitters are employed, some modification is required because of the 

 lack of discreteness to the exposure period. The duration of exposure will depend 

 upon a combination of the radioactive half-life, metabolic activity, and rate of excretion. 

 Continuous exposure to internal emitters at a constant dose rate can be accomplished 

 by the proper adjustment between input of dose increment and decay and excretion 

 rates. 



TECHNIQUES OF EXPOSURE 



Although the irradiation of an animal initially may seem a simple, straightforward 

 procedure requiring little more than the placement of the creature under an X-ray 



