GENETIC TYPE AXD THE ENDOCRINES 353 



condition so common in the bulldog; while the most frequent 

 expression in the F2 hybrids is varying degrees of prog- 

 nathism in the upper jaw, this causing the upper incisors 

 to project far in front of the lower. The prognathism of the 

 maxilla is often so extreme that even when the mouth is closed 

 the tongue protrudes and hangs down in front of the chin. 

 The long upper jaw in such specimens is apparently inherited 

 in independent fashion from the dachshund line while the 

 lower jaw is short and broad through a dominance of the 

 Pekingese. In such an animal the misfit and disharmony of 

 the jaws make feeding from shallow pans difficult, since the 

 nose strikes the pan far in advance of the biting end of the 

 short lower jaw. 



In these crosses the genetic factors influencing growth 

 in length of the mandible are transmitted quite independent 

 of those influencing length of the maxilla and the basicranium. 

 The two latter characters, however, are genetically linked, 

 at least to a large extent. The independence in growth of 

 the upper face to that of the lower jaw seems to be the rule 

 for several widely separated dog breeds. 



When long and short muzzled breeds are crossed, the mem- 

 bers of the first hybrid generation show only mild degrees 

 of disharmony between the upper and lower jaws, probably 

 due to the complex genetic basis and only partial dominance 

 of the factors for determination of either long or short jaws. 

 Members of the second generation, through a recombination 

 of factors separately influencing the length of each .jaw, 

 may show four possible combinations. In the first place, both 

 jaws may be long. Secondly, both may be short and struc- 

 turally harmonious with normal dental occlusion. A third 

 condition is a short maxilla in association with a long under- 

 shot mandible, a disharmony of structures causing complete 

 dental malocclusion. Finally, a fourth condition presents 

 the reverse arrangement long maxilla in association with 

 short reduced mandible, the overshot disharmony with again 

 complete dental malocclusion. 



