66 CHARLES R. STOCKARD 



modified as far as later genetic influence is concerned on 

 account of this close contact f We have made breedings which 

 bear on this question. 



In one case, a pure shepherd bitch was mated to a hetero- 

 zygous bassethound-foxhound male and produced two long 

 legged and two short legged offspring, as was expected on 

 the chance basis. A long legged daughter (plate 8, fig. 2) is II 

 for leg length, but one of the allels for long in this animal's 

 cells was previously associated with the allel for short in the 

 chromosomal pairs of her bassethound-foxhound father. Has 

 the former contact with the s gene affected half the genes for 

 long contained within this daughter! To answer this question 

 the bitch was mated to a pure shepherd male, and nine 

 puppies were whelped, all of which lived to adulthood (see 

 lower line of dogs in plate 8). Every one of these animals 

 had long legs and there was no indication of decrease in leg 

 length, as may have occurred in half of them had the gene 

 for long been contaminated in the heterozygous short legged 

 maternal grandfather. The experiments previously discussed 

 show that the gene for achondroplasic short leg, s, is dominant 

 in its influence over the gene I for long legs, and neither the 

 I nor the s gene is modified by allelic association with the 

 other. Yet each of these genes probably exerts an influence 

 when in the other's presence. As evidence of this last state- 

 ment, it has been mentioned above that individuals which are 

 homozygous for short legs, ss, develop more extremely short- 

 ened legs than do those animals which are mixed or hetero- 

 zygous, si. In other words, a single gene in the mammalian 

 constitution may exert a profound modification in the de- 

 velopment of long bones and decidedly modify the animal 

 type. The presence of two similar allels for inducing the 

 achondroplasic condition gives a more enhanced effect than 

 does only one gene. The enhancement may be due to a sum- 

 mation of effects from the two genes in the homozygous 

 case, but, on the other hand, the weaker achondroplasic effect 

 in the heterozygous animal may be due to some competition 

 with the gene I for normal leg length. These reactions are 



