COILING 

 RIGHT LEFT 



Rr 



Rr 



F, 



lRR+2Rr+lrr 



1 



£ s£i 



3 Families 

 all right 



1 Family 

 ALL left 



Fig. 45. — The direction of coiling of the shell in Liimica pcrc^ra is determined by 

 a single gene difference, the allelomorph giving right-hand coiling being dominant 

 to that giving left. The direction of coiling reflects, however, not the snail's ov^n 

 genotype but that of its mother. Thus all the offspring of a female have the same 

 direction of coiling, even though she be a heterozygote. The segregation will appear 

 in the phenotype as differences between the families of grandchildren. 



In this way the reciprocal F^'s differ in the cross between true breeding strains 

 of right and left coiled snails. The F2's from these reciprocals, however, are alike 

 and are also uniform. The segregation among the genotypes of the F.j's shows 

 itself only in the F3 generation which consists of 3 families of uniformly right 

 coiled snails to i of uniformly left coiled snails, no matter which way the original 

 cross was made. 



192 



