GENETIC TYPE AND THE ENDOCEINES 



67 



so pronounced that the observer may diagnose the limb con- 

 ditions resulting from the influence of two genes as contrasted 

 with the conditions due to only one such gene. The two 

 groups, short and intermediately short, are so clearly dis- 

 tinguishable, and the number of animals making up each 

 group follows so exactly Mendelian expectations, that it is 

 highly probable that the very short group is homozygous 

 and the group with less deformed legs is heterozygous. The 

 crucial proof for the correctness of these suppositions is the 

 genetic test which has been applied in a number of cases. 



Plate 9 illustrates the results of such a genetic test. Figures 

 1 and 2 show two Fj shepherd-bassethound dogs with achon- 

 droplasic legs of intermediate length, the mixed heterozygous 

 condition. At the left in figure 3 is an F L , animal, 560 S , with 

 very short legs, which was diagnosed as homozygous ss. At 

 the right in figure 3 is a long legged, //, F 2 litter mate sister, 

 562 9 . To prove our diagnosis correct, a mating between this 

 brother and sister should produce only offspring with inter- 

 mediate-short legs. The mating was made and ten F 3 puppies 

 were whelped, all having intermediate length short legs. 

 Figures 4, 5 and 6 of plate <) show eight members of this 

 litter which lived to adulthood, and the heterozygous short 

 state of the legs is clearly comparable with the condition 

 shown by the F, hybrids (figs. 1 and 2). This mating thus 

 furnished genetic evidence of practical certainty that the F> 

 male 560 (left in fig. 3) is homozygous, ss, for short legs, 

 as we had diagnosed from morphological examination. It is 

 of considerable importance to know that so complex a de- 

 formity as chondrodystrophy of the extremities, involving 

 as it does the length and shapes of bones and muscles as well 

 as other less evident arrangements, arises from the presence 

 of a single gene which is largely dominant in its influence 

 over the normal allel for long legs. Further, we repeat, one 

 is able to diagnose the animal as genetically mixed or pure 

 for the achondroplasic condition simply from the degree of 

 the morphologic reaction. 



