146 CHARLES K. STOCKAED 



genes. These diagnoses have been substantiated in several 

 cases by crucial genetic tests. It is surprising to realize 

 that the influence of either one or two genes acting on so 

 complex a phenomenon as the growth and development of 

 the mammalian extremity will give rise to two different 

 degrees of distortion so readily differentiated. It is equally 

 surprising that so complicated a disturbance as achondroplasic 

 development of the extremities results from only a single 

 mutant or modified gene within the entire hereditary complex 

 (unless it be from several modified genes within the same 

 chromosome). 



Evidence has been presented which indicates that the same 

 modified gene is the basis of achondroplasia of the extremities 

 in all dog breeds carrying this short leg deformity, and that 

 this gene is carried within a comparable pair of chromosomes 

 in the different breeds in which it occurs, even though many 

 of these may have arisen independently centuries ago in 

 widely separated regions of the world. 



Since this defect of the extremities occurs in man and many 

 other mammals as well as in other classes of vertebrates, 

 it is suggested hypothetically that a gene of identical chemical 

 nature may exist in widely different members of the animal 

 kingdom and that this gene plays a consistent role in the 

 development of individuals. 



The possible influence of unusual endocrine secretions in 

 this modified growth reaction, as well as the possibility of 

 primary inheritance of endocrine conditions bringing about 

 the distortions of the extremities as a secondary effect, are 

 reserved for consideration in a later section of this studv. 



