432 MAMMALIAN RADIATION GENETICS 



particularly effects on fertility, involved in an economical operation of 

 the large project. On the basis of mutation rates in Drosophila, few, 

 if any, mutations at the chosen loci were expected in the pilot experi- 

 ment. Some possible ones were, however, observed, and two of these 

 have already been confirmed by breeding tests. Since more data 

 should be available before long, it would be premature to report the 

 mutation rate calculated from the present figures. The results do indi- 

 cate that the large-scale program should yield enough mutations to give 

 a reliable figure on the induced rate. The anxiety involved in having 

 to depend primarily on Drosophila data in the planning of this large 

 project with mice is thus somewhat reduced. 



CHROMOSOME ABERRATIONS 



Much is already known about the induction by x-rays of chromosomal 

 aberrations in the mouse. The pioneer work was done by Snell (14, 15), 

 Snell and Ames (20), and Hertwig (5, 6, 8). Their results, which have 

 been published in detail and have been reviewed by others [for example. 

 Lea (11)], will be presented briefly here in order to provide a background 

 for a few comments about their bearing on the genetic hazards of radi- 

 ation in man. 



When mice are exposed to a heavy dose (for example, in the neighbor- 

 hood of the median lethal dose) of x-rays and then mated to non-irradi- 

 ated animals, it can be shown that the offspring conceived shortly after 

 irradiation fall into at least four distinct groups. One group dies in 

 early embryonic stages. The death of this group is presumably due to 

 chromosomal aberrations ("dominant lethals") some of which, as shown 

 by Brenneke (1), result in cytologically visible abnormalities in the 

 nuclei of cells in the early cleavage stages. The remaining three groups 

 are born alive and all appear normal, but only one group proves to have 

 normal fertility, the other two being sterile and semisterile, respectively. 

 It is probable that there are other groups, including stillborn and living 

 abnormal animals, but these seem to be relatively rare. 



Since the fertile and the semisterile groups breed, they can be tested 

 genetically. The fertile animals, when outcrossed to normal animals, 

 produce normal, fertile offspring. The semisteriles, when outcrossed, 

 yield three main types of progeny. The first tj^pe is comprised of em- 

 bryos that die at various stages in development. The loss of these ac- 

 counts for the reduced litter size, about one-half normal, by which the 

 semisteriles were first recognized. The other two types, which occur in 

 equal numbers, prove to be semisterile and normal fertile. On further 

 testing of these, they turn out to be like the semisteriles and fertiles ob- 

 tained in the first generation following irradiation. Thus, the semiste- 



