424 THE AMERICAN NATURALIST [Vol. LY 



long-eared Fj individuals. Since it occurred in both 

 types of sperm the short-ear gene must be located in one 

 of the autosomes. 



The Bach-cross. — The back-cross between long-eared 

 mice, heterozygous for short ears and short-eared mice, 

 gave 51 long-eared animals to 43 with short ears (the 

 sums of the figures in Tables II and III). On the assump- 



TABLE II 



Back-cboss. Heterozygous Males Crossed to Short-eared Females 



1656 3 1 



tion that the cluinictcr is due to one .i^cno, the ex]UH'ted 

 back-cross ratio is 1 : 1. In a total of 94, the expectation 

 for the two classes would be 47 : 47. The actual numbers 

 obtained fit the calculated sufficiently well to justify the 

 conclusion that short ears depend upon a single pair of 



In conformity with the results of the first test, the back- 

 cross also shows that there is no sex-linkage concerned 

 in the transmission of the new character. The long-eared 

 Fi males (obtained by crossing normal females with 

 short-eared males) were bred to short-eared females. If 

 the new gene were sex-linked, the Fj male would have but 

 one dose " of the allelomorph long ears. It would be 



