No. 634] SHORTER ARTICLES AND DISCUSSION 



459 



±3.8 and in the F 2 generation 113.3 ± 3.0. The combined 

 ratios are 115.9 ± 2.7. The significant excess of males above 

 that found in the inbred non-waltzing stock is a common result 

 of hybridization. The reciprocal cross gives a most interesting 

 contrast. When Japanese waltzing females are crossed w T ith 

 non-waltzing males the sex ratio is 44.0 ± 7.4. This differs from 

 the sex ratio of the reciprocal cross by 8.9 times the probable 

 error of the difference. 



Certain other evidence is obtainable from the results of mat- 

 ing Japanese waltzing females or their female descendants with 

 hybrids of the P, or back cross generation. Such crosses give 

 opportunity for a lethal factor to be transmitted and to ex- 

 press itself. We should expect, therefore, that an excess of 

 females would be obtained in the progeny of such crosses. Such 

 is actually the case, the ratio being 78.7 ± 2.9. The exact ratio 

 to be expected would depend upon the numerical relation of 

 young in a given population, obtained from homozygous normal 

 females, to those from females carrying the lethal factor. 



The size of litters given by Japanese females irrespective of 

 the sire is distinctly smaller than that of litters from non-waltz- 

 ing females. The average of 58 litters from Japanese waltzing 

 females is 3.38 and from non-waltzing females (100 litters) 5.93. 

 The litters from Japanese females are therefore .57 times the 

 size of those from non-waltzing females. This result is in gen- 

 eral accordance with the presence of a sex-linked lethal factor. 

 x 2 test of the frequency distributions of litters in the two cases 

 shows that the odds are less than one in 1,000,000 that they are 

 the same. 



While it is realized that the above evidence is preliminary, its 

 entire consistency and the controlled nature of the material 

 makes it seem likely that the observed figures indicate the pres- 

 ence of what is apparently the first case of sex-linka-ge in rodents 

 and of a sex-linked lethal factor in mammals. The possible ex- 

 ceptions to this statement 2 are to be found in the case of hemo- 

 philia recently reviewed by Whitman 3 and in the possible dif- 

 ference in reciprocal crosses between Epimys rattus and Epimys 

 alexandrinus reported by De lisle and reviewed by de Meijere. 4 



