CHROMOSOME ABERRATIONS IN ANIMALS G87 



P. Hertwig (1941) found that doses of radiation sufficient to inhibit 

 spermatogenesis led to temporary sterility when the supply of mature 

 spermatozoa had been exhausted. When fertility was restored, the 

 newly formed spermatozoa produced litters of approximately normal 

 size. This result suggests either that few chromosomal breaks are 

 induced in the primordial spermatogonia, or that germinal selection in 

 the course of spermatogenesis eliminates most of the cells with chromo- 

 somal aberrations. 



Evidence of differences of sensitivity in germ cells of female mice at 

 different stages of development was presented by Murray (1931). After 

 exposure to 150 r of X rays, all primary follicles disappeared within two 

 days, and no follicles were seen after 43 days. Details of these various 

 experiments and the extensive literature pertaining thereto have been 

 summarized by Glucksmann (1947). 



Another aspect of the general problem concerns the relative sensitivity 

 of male and female gametes. Metz and Boche (1939) found that the 

 chromosomes of the spermatozoon and the oocyte of Sciara responded 

 differently to X rays. Numerous rearrangements were found among the 

 progeny of irradiated males, but none among the offspring of treated 

 females. Subsequent studies have shown that this difference is not a 

 matter of sex, but depends on the condition of the chromosomes at the 

 time of treatment (Crouse, 1950) . In Drosophila, dominant-lethal effects 

 may be induced by irradiation of the eggs. When oocytes are irradiated 

 before fertilization, a higher proportion of the zygotes fail to hatch than 

 when spermatozoa are irradiated with an equivalent dose (Sonnenblick, 

 1940) ; but as Glass (1940) has shown, irradiation of eggs produces many 

 fewer chromosomal aberrations — inversions but no translocations — than 

 irradiation of sperm. In the gnat, Phryne fenestralis, irradiation of 

 oocytes apparently produces only the dicentric single-break type of 

 aberration, whereas irradiation of spermatozoa also produces multiple- 

 break recombinations (Bauer and Lerche, 1943). 



Such comparisons have at times been interpreted as indicating that 

 some of the dominant-lethal effects produced by irradiation of eggs may 

 not be attributable to induced chromosomal alterations (Muller, 1938). 

 Lea (1946) suggested, however, that dominant lethals induced in unferti- 

 lized eggs, as well as in sperm, can be explained by structural alterations of 

 chromosomes, if it is assumed that the probabilities differ in sperm and 

 egg that a breakage end will join with another breakage end in preference 

 to undergoing sister-union. Similar calculations suggest that chromo- 

 some aberrations are largely responsible also for lethals induced in 

 fertilized eggs (see Lea and Catcheside, 1945; Lea, 1946; Haldane and 

 Lea, 1947). It is suggested that if primary breaks occur at random, and 

 if joinable breaks unite at random, differences in the frequencies of viable 

 types of exchange induced by a given dose of radiation in mature sperms 



