genetic type and the endocrines 103 



The Effects of Different Breed Constitutions on the 



Fundamental Growth Pattern of the Limb Skeleton 



as Eesponses to a Single Gene or to a Homozygous 



Allelic Pair 



In our efforts to determine whether gross deviations from 

 normal type are of direct genetic origin or arise indirectly 

 due to developmental modifications in the internal environ- 

 ment, it was deemed desirable to perform the same experi- 

 ments on more than one breed. The employment of several 

 breeds has facilitated the analysis of this complex condition 

 to a great extent and has thrown some light on the significance 

 of the general constitution in modifying reactions to specific 

 responses. The genetically rather simple phenomenon of 

 achondroplasic growth in the skeletons of the extremities has 

 proven very valuable in demonstrating the differences in 

 degrees of expression of a growth phenomenon as related to 

 different constitutional types. The full meaning of the prob- 

 lem is best brought out as we follow it through. 



In connection with the inheritance of dwarf growth in the 

 leg, we have discussed hybrids resulting from crosses involv- 

 ing four different breeds. The dwarf leg of the bassethound 

 was tested against the German shepherd, our standard con- 

 trol, and, toward one extreme, against the long, slender grey- 

 hound typed leg of the Saluki, and in opposite direction 

 against the shorter, stocky typed leg of the bulldog. In figure 

 1 of plate 23 the skeletons of the right front legs from these 



plate 22 



EXPLANATION OF FIGURES 



Second generation English bulldog-bassethound litter mates illustrating differ- 

 ences in leg length and bone type. 



1 and 4 1311 J* long legs (11) on bulldog bone. 



2 and 5 1312^ long legs (11) on incomplete hound bone. 



3 and 6 1313 <$ short legs (ss) on hound bone. 



The pure long legged animal with bulldog bone (figs. 1 and 4) has legs fully 

 as thick as the animal in figures 3 and 6 which is homozygous for achondroplasia 

 but has hound typed bone. 



