GENETIC EFFECTS IN MAMMALS 849 



tion of the results obtained from the matings of partially sterile to par- 

 tially sterile made within two of the lines. Snell and Picken (1935) 

 report a presumed translocation in which the fertility of heterozygotes 

 was almost normal. This case was discovered through the occurrence 

 of a few abnormal offspring. It should be kept in mind that, as has 

 already been mentioned, translocations showing considerably more than 

 50 per cent fertility in the heterozygote would usually not be detected by 

 fertility tests. Present determinations of the average fertility of trans- 

 location heterozygotes may, therefore, be biased. 



The time of death and nature of the abnormal embryos produced by 

 the aneuploid gametes in translocation stocks obtained from irradiated 

 mice have been studied by Snell, Bodemann, and Hollander (1934), Snell 

 and Picken (1935), Hertwig (1938a, 1940), and Otis (1949). These 

 investigations show that the commonest time of death is at implantation 

 or shortly after. There is evidence of some death before this time and 

 also of some survival even to birth. The proportions falling into these 

 classes vary considerably among the translocation lines. Snell and his 

 associates found that failure of the neural groove to close at its anterior 

 end was a common type of defect in the small proportion of abnormal 

 embryos that survive to later stages. 



The expected proportion of partially sterile animals in the viable off- 

 spring of a mating of a translocation heterozygote with a normal is one- 

 half. It would, however, be lower than this if the heterozygotes were 

 less viable than the normals. On the other hand the proportion of 

 normals would be depressed if any aneuploid zygotes were viable. 

 Hertwig (1940), using the mean-litter-size test for partial sterility, found 

 that the percentage of partially steriles in the offspring of such crosses fell 

 below 50 per cent in ten out of eleven translocation lines. The mean for 

 the eleven lines was 45 per cent. In contrast, Snell (1946), who made use 

 of genetic markers to establish the proportion of partial sterility, found 

 no significant departure from a 1 : 1 ratio. In all of five sets of his data, 

 involving three marker genes for one translocation and two marker genes 

 for another, there was actually a slight, though not significant, excess of 

 the classes marked by the gene that entered the cross with partial sterility. 



Translocations have been obtained in homozygous condition in two, or 

 possibly three, of six lines tested by Hertwig (1940) and in both of two 

 lines tested by Snell (1946). The failure to obtain the homozygotes in 

 Hertwig's other lines could be attributed to limitations of the test rather 

 than to lethality of the homozygotes. The bearers of the homozygous 

 translocations proved to be phenotypically normal with the exception of 

 one of Snell's which showed "a suggestion of a reduced viability." Thus, 

 as far as they go, the data for the mouse indicate that position effect may 

 be less important than it is in Drosophila. 



The translocations produced by irradiation in mice are proving increas- 



