242 BIOLOGY OF THE LABORATORY MOUSE 



Vaginal occlusion. — Occurred in a number of individuals of silver strain. 

 Marx, 1936, Anat. Rec. 66: 449-454. 



Induced Chromosome Mutations 



X-rays and neutron rays when applied to mature ova and spermatozoa of 

 mice are a prolific source of translocations.* Other sorts of chromosome 

 mutations probably are induced also, but are not detectable by the genetic 

 methods at present available. The detection of translocations is relatively 

 easy, due to the fact that mice heterozygous for a translocation are semi- 

 sterile, consistently producing small litters even when mated to unrelated 

 and entirely normal mice. 



T-F1146 translocation-F\i46. — Average size of litters from the mating 

 T-Fii46/-\- X +/+ is 4.6 as compared with the normal value for the stock 

 of 8.3. Reduction in litter size is due to death in utero of approximately 

 45% of the embryos. Most of these embryos die shortly after implantation ; 

 a few live to later stages, occasionally even to term, but show brain abnor- 

 malities due to failure of the neural groove to close at the anterior end. 

 Of the viable young, one half on the average are semi-sterile, one half 

 normal. The chromosomes involved are 5 and the chromosome carrying b 

 (0% crossing over between a and break, 20% between break and h). Snell, 

 Bodemann and Hollander, 1934, J. Exp. Zool. 67: 93-104. Snell, 1941, 

 Genetics 26: 169. 



T-i translocation- 1. — Very little reduction in litter size, but almost 

 4.6% of the offspring from the cross T-i/-\- X +/+ show brain abnor- 

 malities due to failure of the neural groove to close at the anterior end. 

 These frequently come to term. The evidence that this is a translocation 

 is not complete. Snell and Picken, 1935, J. Genet. 31: 213-235. 



Rules for Assigning Symbols to Mutations 



The following rules for assigning symbols to mutations have been 

 adopted by the Committee on Mouse Genetics Nomenclature, j 



1. The initial letter of the mutant symbol shall be the same as the 

 initial letter of the mutant gene, e.g., d for dilution. 



2. Additional letters shall be added to the initial letter if necessary to 

 distinguish it from symbols already in use. These shall be, preferably, 

 those immediately following the initial letter, or suggestive letters, espe- 



* Snell, 1935, Genetics 20: 545-567; Snell and Ames, 1939, Am. J. Roent. Rad. 

 Therapy 41: 248-255. 



t Dunn, Gruneberg and Snell, 1940, J. Hered. 31: 505-506. 



