144 CHARLES U. STOCKAED 



No intermediate legs typical of the Fj hybrids between short 

 and tall dogs occur in this cross. A number of matings were 

 made among the F, hybrids and these resulted in twenty-three 

 F 2 puppies, all of which showed uniformly short extremities ; 

 there were no intermediate and no exaggerated forms, and 

 all were of exactly the same type as in the two pure parent 

 stocks (see pi. 67, p. 345). 



This result indicates quite conclusively that the modified 

 extremities in these breeds are due to a mutation which 

 affected the same gene in both breeds, and further, that at 

 the present time this gene is located in the same pair of 

 chromosomes in each breed. This is significant since the two 

 breeds arose long ago in widely separated parts of the world. 



On the basis of the facts obtained from the many breed 

 crosses considered for leg length, it would seem possible 

 that all animals in which the achondroplasia defect of the 

 extremities occurs, including man himself, must possess this 

 same specific gene. There is a possibility that a given gene 

 consists of one or more specific protein molecules and thai 

 the same gene or same protein molecule might occur in a 

 great number of animal species. Further, it is possible that 

 in the various species of animals in which achondroplasia 

 occurs, a mutation of one specific gene common to all gave 

 rise to modified extremities in the mutant individual. Only 

 by investigating some such suppositions as these can we 

 attempt to understand why the same type of modification 

 can be inherited in almost all higher classes of vertebrates. 

 The fact that in these problems our chemical knowledge is 

 very far behind should not discourage investigation of the 

 genetics and development of these characters. Such studies 

 may in the end lead the way to a knowledge of the chemistry 

 involved. Certainly the chemist cannot hand the biologist 

 the composition of chromosomes and genes until he is directed 

 by the biologist to the sources of material. Biological assays 

 and reactions were necessary before the chemist could start 

 an approach to thyroxin or oestrin (theelin). The perform- 

 ance of the chemists in the solution of biological problems 



