Cannon and Rosanoff : Heredity of Insanity 3 



ing the determiner for brown eye pigment from one parent only) , 

 the other blue-eyed: half the children will be brown-eyed and 

 simplex and the other half blue-eyed : DR X RR «> DR -f RR. 



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

 the children will be brown-eyed and simplex: DD X RR <» DR. 



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 co DD -f 2DR -f RR. 



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

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

 DD X DR 00 DD + DR. 



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

 be brown-eyed and duplex : DD X DD 00 DD. 



It will be readily seen from these formulae that in attempting 

 to predict the proportions of 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 case of eye color, an individual with blue 

 eyes 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 inher- 

 ited the determiner for that character from both parents or only 

 from one? We can judge this only by a study of the ancestry 

 and offspring of the individual. 



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

 between a dominant and a recessive condition of a character is in 

 the fact that in a case of simplex inheritance the dominant condi- 

 tion is plainly manifest, while the recessive condition is not appar- 

 ent and can be known to exist only through a study of ancestry 

 and offspring. 



This is important because it constitutes the criterion by which 

 we are able to determine whether any given inherited peculiarity 

 or abnormality is, as compared with the average or normal condi- 

 tion, dominant or recessive. 



§ 2. General Survey of Material. — We may now proceed with 

 the examination of our material, which consists of the pedigrees 



