CHROMOSOME ABERRATIONS 435 



shows that the incidence of semisterility is high only in offspring sired 

 shortly after irradiation, namely, before the beginning of the sterile period 

 that follows within a few weeks after the giving of high acute doses. In 

 the progeny conceived after the end of the sterile period, the incidence 

 of sterility and semisterility did not differ significantly from that in the 

 controls. Applying these results to man, the important practical con- 

 clusion can, therefore, be drawn that the probability of passing on cer- 

 tain types of chromosomal aberrations to the next generation will be 

 greatly reduced if individuals exposed to high doses of radiation refrain 

 from begetting offspring for a period of perhaps 2 or 3 months after ex- 

 posure. 



Another problem concerning translocations that is of practical impor- 

 tance in man is the determination of the exact times of death of the an- 

 euploids, or unbalanced chromosomal types, which result from the mat- 

 ings of semisterile with normal individuals and which apparently perish 

 while still in embryonic stages. The death of an advanced fetus would 

 obviously constitute more of a hazard to the mother than the loss of an 

 early embryo. In the mouse, there seems to be considerable variation 

 in the time of death, but from the work done so far, including the study 

 by Otis (13), who also made a careful comparison of normal mouse and 

 human developmental stages, it may be tentatively concluded that most 

 deaths would occur before the seventh week of human pregnancy. It 

 must be remembered, however, that the number of semisterile lines in 

 the mouse that has been examined for this characteristic is not yet 

 large. 



There are many other aspects to the problem of induced chromosomal 

 aberrations in mice which require further study. For example, in two 

 out of eleven semisterile lines being tested at Oak Ridge, the viable prog- 

 eny of outcrosses of semisterile with normal do not fit the simple ratio 

 of one-half semisterile and one-half fertile. Both these lines are pro- 

 ducing some completely sterile descendants, and one is yielding individ- 

 uals whose fertility is apparently well below that of characteristic semi- 

 steriles. It is not yet clear what the pattern of inheritance is in these 

 lines. 



The possible effects of translocations in individuals homozygous for 

 them also need to be investigated further. The few cases obtained so 

 far are apparently completely normal phenotypically. This indicates, as 

 Hertwig (8) pointed out, that position effect may be relatively unim- 

 portant in the mouse as compared with Drosophila. 



Very little is known about the induction of chromosomal aberrations 

 in mammals by radiations other than x-rays. Snell and Aebersold (19) 

 and Snell (16) made a few experiments with neutrons. Dosage compari- 



