INHERITANCE IN MANKIND 37 



Such a character has been found by Hurst hi the 

 brown pigment m front of the iris of the eye. When 

 present in large amount, this pigment gives to the eye 

 the appearance known as brown or black, while, if there 

 be little or no pigment, the eye is described as blue or 

 grey. 



Hurst found that the type of eye possessing the 

 brown pigment was dominant to that without it ; that 

 the factor giving brown pigment, if present at all, 

 always showed in the eye. We may say then that 

 brown eyes are dominant and blue eyes recessive ; 

 though casual description may lead to error, as it is 

 the presence of the pigment which is the Mendelian 

 factor, while the general appearance of the eye depends 

 on the amount of the pigment. 



If we represent the dominant character by D and 

 the recessive by R, the mating of two individuals, one 

 pure- bred with regard to brown eyes and the other 

 pure- bred with regard to grey, may be denoted by 

 DD = RR. Since the children must obtain one factor 

 from each parent, they v/ill all be represented by DR, 

 and in appearance all will resemble the dominant parent 

 — that is, all will possess brown pigment in their eyes. 



If one of these offspring marry a pure-bred grey- 

 eyed person, an RR, we have the union DR = RR. 

 On the average, half the children of such marriages will 

 be DR and half RR. The former class will be brown- 

 eyed, but not pure-bred, while the latter class will be 

 pure recessives who have lost altogether the factor on 

 which brown eyes depend. As long as they mate with 

 grey-eyed people their descendants will never have 

 brown eyes. 



