GENETIC TYPE AND THE ENDOCRINES 87 



The shortness of legs in the three intermediates at the left 

 is more pronounced than in the intermediates shown in 

 plate 14, and abduction of the foot is more pronounced in 

 the intermediate backcross bassethound than in the inter- 

 mediate short legs of the backcross Saluki. Both these 

 differences in growth expression are due to differences 

 in relative completeness of the dominant Saluki typed 

 bone which seems to be a multiple factor character. Although 

 there is some indication of certain dominant factors for 

 Saluki type even in the backcross bassethounds, yet this type 

 is not nearly so complete as in the backcross on the Saluki 

 itself. These facts indicate that although the Saluki con- 

 stitution for bone quality is quite dominant in this cross, 

 complete dominance depends upon more than one influencing 

 factor rather than upon only a single genie reaction, as is 

 the case in the transmission of chondrodystrophic growth. 



A second backcross with a different F x male, 505, and a 

 different pure bassethound bitch, 277, whelped a litter of 

 four puppies. These showed conditions closely similar to 

 those discussed above. 



A further test mating using these backcrosses was made 

 in an effort to determine as clearly as possible whether the 

 heterozygous condition, that is the presence of only one gene 

 for chondrodystrophic short leg, gives a morphological ex- 

 pression of this distortion differing to a clearly distinguish- 

 able degree from the expression resulting from a pair of 

 genes or the homozygous condition for such growth. In other 

 words, in so complex a phenomenon as the dwarfing in skeletal 

 development of the limb in higher mammals, can differences 

 be consistently recognized which are simply due to the pres- 

 ence of either the single gene or the pair of genes determining 

 this dominant condition? Since the possibilities for varia- 

 tions are not so great in the backcrosses, the leg conditions 

 of intermediate si and double ss should be morphologically 

 distinguishable with somewhat more certainty than they are 

 among the F 2 hybrids. 



