GENETIC TYPE AND THE ENDOCRINES 389 



screw-tail (skeleton fig. 6, pi. 20). It should be observed 

 that this screw-tailed F 2 bitch, 918 $ , has long, straight legs 

 and a strong, well developed, hound-like head. The screw-tail 

 evidently developed in an endocrine environment which was 

 sufficiently normal to permit a proper pattern of growth in 

 all other body parts, the deformity of tail being directly 

 due to the presence in this dog's genetic composition of the 

 two recessive genes for short tail and the two recessive allels 

 for bent tail characters. One could scarcely imagine this 

 screw-tail to have resulted from a localized endocrine attack 

 affecting only certain vertebrae of the tail. 



The impossibility of such an explanation becomes more 

 evident on examining the tail conditions in the backcross 

 hybrids between the F l bulldog-bassethound and the pure 

 bulldog stock. Here, of course, there is larger chance for the 

 occurrence of the screw-tail condition than there is among 

 the F2 hybrids. 



An examination of a large number of F 2 and backcross 

 hybrids shows the following kinds of tails : long-straight, 

 short-straight, long-bent, short-bent, and screw-tail. We may 

 represent the genes concerned with these tail characters by 

 the letters L for long, St for straight, s for short and b 

 for bent. The screw-tailed bulldog would have the homo- 

 zygous composition ss-bb, being pure for short and pure for 

 bent, and only this particular genie arrangement can give 

 rise to the extreme condition. The Fj hybrid is heterozygous 

 for both factors, Ls-Stb, and develops the dominant long- 

 straight tail. 



The sixteen possible genie combinations for the F 2 genera- 

 tion are shown in text-figure 78. As indicated in the squares, 

 these combinations give rise to a definite distribution of tail 

 patterns. Nine long-straight tails resulting from the com- 

 bined presence of the dominant genes L and St, and three 

 long-bent tails due to the combination of the dominant L 

 for long tail with the double recessive bb for bent, appear. 

 Theoretically, there should be three short-straight tails re- 



