GENETIC TYPE AND THE ENDOCEINES 143 



droplasia in the bassethound and the dachshund were quite 

 certainly carried by comparable pairs of chromosomes. 



Because both above breeds are European in origin, the 

 mutation giving rise to achondroplasia extremities may have 

 appeared in the germ cells of ancestors common to both of 

 them and on this account it was deemed desirable to conduct 

 an experiment with breeds derived from two widely distant 

 parts of the world as had been done in the previous con- 

 trasted crosses. We again used the European dachshund, 

 this time crossing it with the Asiatic Pekingese dog. There 

 is no indication of any relationship whatsoever between these 

 two breeds as such. Therefore the achondroplasia leg char- 

 acter found in both breeds very probably arose through in- 

 dependent mutations. The question is again presented: does 

 this character always depend upon a mutation which affects 

 the same gene? And if it does, is this gene always located 

 in the same chromosome in all the breeds involved? It has 

 been clearly shown by Dobzhansky ('37) for the fruit fly 

 that a given gene is not always carried in the same chromo- 

 some in all varieties of these flies. Whether or not this is 

 true for the germ cells in the higher animals, and particularly 

 in mammals, no one has determined. At present the only 

 way to detect such a discrepancy in the location of a given 

 gene in different varieties or breeds of a mammal is through 

 crossing widely separated breeds that chance to show the 

 same single point mutation. 



The dachshund and the Pekingese both have achondroplasia 

 of the extremities which w r e have demonstrated above to be 

 transmitted independently as a single factor dominant char- 

 acter. These breeds were crossed a number of times in both 

 directions and the details of these crosses will be discussed 

 in other connections. At this time we shall merely summarize 

 briefly the results as they concern the growth of the hybrid 

 extremities. 



Fifteen F x dachshund-Pekingese hybrids were produced 

 and all showed fully short achondroplasic extremities; five 

 members of this generation may be seen in plate 67 (p. 345). 



