GENETIC TYPE AND THE ENDOCRINES 145 



must necessarily lag behind the discovery of the problems. 

 Not only is this difference in time of performance evident 

 in the interdependence of two different sciences, such as 

 biology and chemistry, but the same may be true for two 

 related fields of the same science. Those familiar with the 

 progress of genetics will know that our knowledge of the 

 cytology of germ cells continually lagged behind genetic 

 discoveries until, it may be said, genetics finally pulled 

 cytology into its own. 



After the foregoing survey of the genetic behavior of the 

 achondroplasia distortion of the extremities, we may finally 

 epitomize the facts regarding this condition as follows: 



Achondroplasia extremities occur among a number of mam- 

 malian species, including man, as an isolated or localized 

 structural deformity in otherwise normal individuals. 



This condition exists in several of the domestic breeds of 

 dogs from widely distant countries, and in all cases where 

 animals showing the condition arc crossed with those having 

 normal extremities, the distorted growth is inherited as a 

 single factor dominant character. 



The influence of the factor for leg achondroplasia differs 

 in degree of severity depending upon the bone constitution 

 of the breed concerned. The Saluki or greyhound typed bone 

 has the greatest resistance to this growth distortion, the 

 shepherd dog bone is affected to a somewhat greater degree, 

 the hound bone to a still greater degree, and the bulldog 

 bone gives the most pronounced response. 



The presence of only a single gene in the heterozygous 

 genotype for limb achondroplasia gives rise to an incomplete 

 or intermediate degree of expression in the phenotype, and 

 the homozygous condition, with both allelomorphic genes 

 for achondroplasia, brings about a complete or full expression 

 of the defect. On the basis of morphologic expression alone, 

 the observer is able to diagnose the difference between the 

 mixed or heterozygous animal carrying only one gene for 

 the condition and the pure homozygous animal with two 



