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THE AMERICAN NATURALIST [Vol. LIV 



Inasmuch as only a few sex-linked factors of any sort are 

 known in laboratory mammals where numerous other genetic 

 differences have been recorded, breeding tests of linkage rela- 

 tions by ordinary methods are precluded. The first indications 

 of a sex-linked lethal factor would therefore be expected to con- 

 sist of an abnormal sex ratio at birth and a reduction in the size 



Such conditions are met with in certain Japanese waltzing mice 

 derived from a remarkably closely inbred race. This particular 

 strain of Japanese mice has been inbred from the descendants 

 of a single pair of animals for approximately fourteen years. The 

 animals are difficult to raise and the litters are frequently dis- 

 tinctly smaller than those of non-waltzing mice of inbred races. 

 The great majority of animals of this strain have been bred by 

 Mr. George Lambert of Boston, who has provided adult or young 

 adult animals to the writer for several years. Mr. Lambert is 

 now engaged in recording sex ratio data at the birth of litters 

 and states that at present an excess of females is being produced 

 in his stock. While awaiting increased numerical data from his 

 records, however, it appeared advisable to make a short note of 

 the sex ratio and behavior of this strain in the animals raised 

 under controlled observation. 1 The sex ratio of inbred non- 

 waltzing mice is 103.1 ± 2.8, showing a slight excess of males. 

 The sex ratio obtained from Japanese waltzing race of the Lam- 

 bert strain is 53.2 ± 5.7. The difference between these ratios is 

 7.9 times its probable error and is certainly significant. 



Since recessive Sex-linked lethal factors are transmissible 

 through a female to one half of her male offspring, the sex ratio 

 among the progeny of females carrying sex-linked lethal factors 

 should be theoretically 50.0. Males of the Japanese waltzing 

 race can not, of course, transmit the lethal factor, since if they 

 possessed the factor it would result in their death. If then ani- 

 mals of the Japanese waltzing race are crossed with non-waltz- 

 ing mice of a race presumably free from lethals, the reciprocal 

 crosses should give markedly different sex ratios. Such is actu- 

 ally the case. Non-waltzing females by Japanese waltzing males 

 should give a sex ratio free from the effect of lethal faotorst 

 The observed sex ratio in the F x generation of this cross is 118.2 

 ^iDr. H. J. Bagg, of the Memorial Hospital, New York City, has kindly 



own in this publication". 



