ALTERNATIVE INHERITANCE 117 



which the other lacks, the offspring get a corresponding deter- 

 miner x from one side only, and when the germ-cells are formed 

 in that offspring half of them are supposed to have the determiner 

 x and half not. This hypothetical process is called the segrega- 

 tion of determiners, and experimental results suggest its reality. 



"The characteristic in the offspring that is due to a single 

 (instead of the normal double) determiner is called a simplex 

 characteristic. Such a characteristic is frequently distinguish- 

 able from one that is due to the double determiner by its imper- 

 fect development. Thus the offspring of a pure black-eyed and 

 a blue-eyed parent will have brown eyes. 



" It is a corollary of the foregoing that if the individual with a 

 simplex character be mated to one lacking the character, half of 

 the offspring will lack the determiner and half will be simplex, 

 again, in respect to the character. If in both cases the character 

 be simplex, the two like determiners will meet in one fourth 

 of the unions of egg and sperm, the two will both be absent in 

 one-fourth of the unions, and one only will occur in half of the 

 unions such will be simplex again. If one parent have the 

 characteristic simplex and the other duplex, then half of the 

 offspring will have it simplex and half duplex. 



" Starting with the principles just enunciated, we reach at once 

 the most important generalisation of the modern science of 

 heredity : " When a determiner of a characteristic is absent from the 

 germ-plasm of both parents (as proved * by its absence from their 

 bodies) it will be absent in all of their offspring " (Eugenics, 1910, 

 pp. 8-9). 



To illustrate the precision f with which the characteristics of 

 offspring may be predicted in the best-studied cases, Davenport 

 refers to eye-colour. 



* Perhaps " inferred " would be a more accurate word than " proved.'' 

 What is inherited in the germ-plasm is not necessarily expressed in de- 

 velopment. 



t See, however, Galloway (1912). 



