224 A STUDY OF HEREDITY IN INSANITY [Oct. 



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

 ing the determiner for brown-eye pigment from one grandparent 

 only), the other blue-eyed : one-half of the children will be brown- 

 eyed and simplex and the other half blue-eyed : 



DRxRRooDR + RR. 



3. One parent brown-eyed and duplex, the other blue-eyed : 

 all the children will be brown-eyed and simplex: 



DDxRRooDR. 



4. Both parents brown-eyed and simplex: one-fourth of the 

 children will be brown-eyed and duplex, one-half will be brown- 

 eyed and simplex, and the remaining one-fourth will be blue- 

 eyed (nulliplex) : 



DR X DR 00 DD -h 2DR -1- RR. 



5. Both parents brown-eyed, one duplex the other simplex: 

 all the children will be brown-eyed, half duplex and half simplex : 



DDxDRooDD+DR. 



6. Both parents brown-eyed and duplex: all the children will 

 be brown-eyed and duplex: 



DDxDDcoDD. 



It will be seen from these formulae that in attempting to pre- 

 dict the various types of offspring that may result from a given 

 mating it is necessary to know not only whether the character is 

 in each parent dominant or recessive, but in the case of the 

 dominant condition also whether it is duplex or simplex. 



Turning "again to the example of eye color, a blue-eyed indi- 

 vidual we know to be nulliplex, as he has no brown pigment 

 in his eyes and therefore could not have inherited the determiner 

 for brown-eye pigment from either parent. But how are we to 

 judge in the case of a brown-eyed person whether he has in- 

 herited the determiner for that character from both parents or 

 only from one? We can judge this only by considering the 

 ancestry and offspring of the individual. 



To put the whole matter in a nutshell, the essential difference 

 between the dominant and the recessive conditions of a character 

 lies in the fact that in a case of simplex inheritance the dominant 

 condition is plainly manifest, while the recessive condition is not 

 apparent and can be known to exist only through a study of 

 ancestry and offspring. 



