GENETIC TYPE AND THE ENDOCRINES 101 



The skeleton of 917 6 (fig. 5), with double short ss on hound 

 typed bone, is no shorter than the two single short F^ on 

 bulldog' typed bone. 



This litter shows both tall and short animals, with hound 

 typed and bulldog typed bone in both classes. Bone constitu- 

 tion or type is an important element in influencing the length 

 of legs in hybrids and, further, is significant in effecting the 

 degree of expression in achondroplasia growth. 



Three male P 2 animals are illustrated in plate '22. No. 1311 $ 

 (figs. 1 and -4) has long bulldog bone legs, and 1312 3 (figs. 

 2 and 5) has long legs with incomplete hound typed bone. 

 Animal 1313 6 (figs. 3 and 6) is double short for achondro- 

 plasia on hound typed bone. Deformed legs on this type of 

 bone are no thicker than the long legs of the brother with 

 bulldog typed bone (figs. 1 and 4). 



The Fj bassethound-bidldog backcrossed with each of the 

 parent stocks. Three fertile matings were made between F T 

 hybrids and the bassethound parent stock. These matings 

 produced litters of seven, six and nine puppies, a total of 

 twenty-two. The results from this backcross, as concerns 

 leg growth, were exactly as would be expected in view of the 

 foregoing crosses. The individuals in this generation have 

 short, achondroplasia legs; about half of them show the con- 

 dition to a limited degree and half have fully deformed short 

 legs. 



Four backcross matings were made in the opposite direc- 

 tion, that is, on the bulldog parent stock. These matings gave 

 litters of three, five, seven and six puppies, a total of twenty- 

 one. About half these animals had long, straight legs of bull- 

 dog type and half had very short, achondroplasia legs. 

 Although these short legged animals are heterozygous for 

 leg length, the deformity is highly marked in its expression 

 because of the dominant bulldog quality of their bone. The 

 significance of bone quality in these growth reactions has 

 been referred to in several connections, but the relation 

 of differences in types of bone constitution to differences 

 in growth reaction is more fully discussed in the chapter 

 following. 



