GENETIC TYPE AND THE ENDOCRINES 53 



nine pups in a litter. The female maternal reactions are very 

 good, and all puppies are usually reared to adulthood. 



These F! hybrids are surprisingly uniform in their char- 

 acters and are mistaken for a standardized breed even by 

 persons familiar with dogs. This uniformity in appearance 

 of a first hybrid generation can only result from crosses 

 between two very pure parent stocks. If the original stocks 

 are not homozygous for their breed characteristics, variations 

 in the qualities of the F! hybrids will appear as indications 

 of the impurity in the parent stocks. Thus the uniformity in 

 qualities of the F l generation is the crucial test for the purity 

 of the parent breeds involved. 



Returning to the special problems related to the inheritance 

 of achondroplasia in the legs, we may now consider this 

 character in the F 2 generation produced from inter se matings 

 among the uniformly typed F x hybrids. Twenty-three fertile 

 matings of F l hybrids have whelped 144 puppies during a 

 period of 8 years. Approximately one-fourth of these F2 

 hybrids have long, straight legs with no symptoms of achon- 

 droplasia and three-fourths have typical achondroplasic short 

 legs. On further examination the short legged animals may 

 be separated into two groups, the smaller number having 

 extreme achondroplasia with very short legs and a larger 

 group, about twice as many, having achondroplasic legs that 

 are less shortened and show almost exactly the same condition 

 as that found in the F x parents. The F2 generation is thus 

 divided into three leg-length groups : one-fourth have very 

 short and badly twisted extremities, one-fourth have normal 

 long legs, and half are intermediate in leg condition. In 

 other words this is the typical 1 :2 :1 Mendelian ratio for the 

 behavior of contrasted expressions of a given character which 

 depends for its inheritance on a single point genetic factor 

 or one gene. Achondroplasic growth of the leg bones is 

 dominant over normal long bone growth and results from 

 the presence of a single allel for this expression. In the 

 heterozygous or mixed condition the influence of the allel 

 for achondroplasia is less pronounced than when the two 



