GENETIC TYPE AND THE ENDOCRINES 109 



is that the entire complex for the bone constitution typical 

 of a given breed determines the degree of response to the 

 dominant gene underlying the achondroplasic reaction. In 

 other words, the genetic complex which causes the pure bred 

 Saluki bone to express the greyhound-like type in contrast 

 with the shepherd type also causes the Saluki bone to respond 

 to the achondroplasic factor to a lesser degree than does the 

 shepherd bone. One might say that the Saluki bone is further 

 away from the achondroplasic defect than is the shepherd 

 bone, and that it is therefore more difficult to turn its growth 

 reaction toward such a pattern. 



The homozygous ss achondroplasic legs which appear among 

 the F 2 hybrids from these crosses furnish very important 

 material in the elucidation of the above questions. In the 

 first place, we have proven genetically in the foregoing ex- 

 periments that the very short achondroplasic legs in the F 2 

 groups are possessed by animals homozygous, ss, for the short 

 condition. In these animals the allelomorph I for long legs 

 is completely absent and therefore cannot enter into modifica- 

 tion in the degree of the action of the gene for achondroplasic 

 legs. Should all the F 2 dogs homozygous, ss, for achondro- 

 plasia not show exactly the same condition in their leg bones, 

 the variations must be due to differences in interaction be- 

 tween the genes for achondroplasia and the breed constitutions 

 of the bones concerned. 



The first three leg skeletons of figure 3 in plate 23 are 

 from F 2 hybrids homozygous for leg achondroplasia, and the 

 fourth leg skeleton is that of a pure bassethound, placed 

 here for comparison. The skeletons are placed in the same 

 order as in the first two groups and are, from left to right, 

 bassethound-Saluki, bassethound-shepherd, bassethound-bull- 

 dog and pure bassethound. The F 2 bassethound-Saluki skele- 

 ton w r as selected as showing full achondroplasia ss upon 

 Saluki typed bone. In spite of the homozygous condition for 

 short, the bones of this leg are slenderer and more graceful 

 than those in the other three skeletons. The scapula is not 

 quite so squared and the humerus is thinner for its length 



