No. 519] THE IMPERFECTION OF DOMINANCE 



light and it has been said that they show a reversal of 



ness on its absence. ( Mearly a given character can hardly 

 be now due to a determiner and again to its absence. The 

 more reasonable hypothesis is that a determiner, though 

 present, may fail to complete its ontogeny. 



An insight into the cause of this failure is given by 

 modern studies in cytology and bleeding. These indicate 

 that, ordinarily, in pure races, a well-developed, pure- 

 bred character has a double determiner as its embryo- 

 logical anlage, while in the heterozygote the determiner 

 is simplex. Now in just these cases when the anlage is 

 simplex the character develops imperfectly; and it is not 

 difficult to understand how, under certain circumstances, 

 the simplex determiner might be insufficient for the de- 

 velopment of the organ. Such would result in complete 

 failure to dominate, not a reversal of dominance. 



Further proof that dominance is not reversed, but only 

 weakened, is derived from the facts of retardation in the 

 ontogeny of some heterozygotes. For example, the white 

 of the Leghorn is dominant over black, but, in the females 

 at least, the dominance is so imperfect that the plumage 

 of the hybrid shows many black spots or is spangled or it 

 may be of a uniform blue; but in later molts it becomes 

 pure or nearly pure white. Similarly. Lang found that 

 the hybrid snails between the red and yellow forms some- 

 times showed at first the recessive yellow, but later gained 

 the dominant red. The ontogeny of the heterozygote 

 characteristic thus appears retarded and frequently fails 

 altogether to reach its final goal. 



This failure of the simplex determiner of the hetero- 

 zygote to develop the corresponding characteristic is 

 frequently found in one sex and not in the other. For 

 example, the simplex determiner for horn production in 

 sheep suffices to induce horns in the males but not in the 



