2 Eugenics Record Office, Bui.i,etin No. 3 



The dominant and recessive conditions of a character are often 

 designated by the symbols D and R, respectively. 



To make the matter clearer, we may take as an example of a 

 Mendelian character the case of eye color. 



The brown color is the dominant condition, while the blue 

 color is the recessive condition, as w^as shown by Davenport.^ It 

 would seem that the inheritance of brown eyes is due to the pres- 

 ence in the germ plasm of a determiner upon which the formation 

 of brown pigment in the anterior layers of the irides depends. 



On the other hand, the inheritance of blue eyes is believed to 

 be due to the lack of determiner for brown eye pigment in the 

 germ plasm; for the blue color of eyes is due merely to the ab- 

 sence of brown pigment, the effect of blue being produced by the 

 choroid coat shining through the opalescent but pigment-free 

 anterior layers of the irides in such cases. 



It must be borne in mind that as regards the condition of any 

 character every person inherits from two sources, namely, from 

 each parent. Therefore, with reference to any character he may 

 be pure bred or hybrid. 



A case of inheritance of a character from both parents is 

 spoken of as one of duplex inheritance and is often designated by 

 the symbol DD. 



A case of inheritance of a character from only one parent is 

 spoken of as one of simplex inheritance and is designated by the 

 symbol DR. 



A case in which a character is not inherited from either parent, 

 therefore exhibiting the recessive condition, is spoken of as one 

 of nulliplex inheritance and is designated by the symbol RR. 



We are now in a position to estimate the relative number of 

 each type of offspring according to theoretical expectation in the 

 case of any combination of mates. 



There are but six theoretically possible combinations of mates. 

 Continuing to make use of the case of eye color as an instance of 

 a Mendelian character, let us consider in turn each theoretical 

 possibility. 



1. Both parents blue-eyed (nulliplex) : all children will be 

 blue-eyed, as may be shown by the following biological formula: 

 RR X RR 00 RR. 



2. One parent brown-eyed and simplex (that is to say, inherit- 

 ' Science, N S., Vol. XXVI, Nov. i, 1907, pp. 589-592. 



