THE METHOD OF EUGENICS 19 



DETERMINERS — Continued 

 Case One parent Other parent Offspring Characteristics of offspring 



3 PP pp Pp, Pp All pigmented and all simplex 



4 Pp I^ PP, Pp, pP, pp }4 duplex pigmented; }4 sim- 



plex; J^ unpigmented (blue- 

 eyed) 



5 Pp pp Pp, PP 3^ simplex; 3^ unpigmented 



(blue-eyed) 



6 pp pp pp, pp All unpigmented (blue-eyed) 



In the case of an individual who has received the deter- 

 miner for one of his unit characters from one side of the 

 house only (say from mother), not only is the character 

 simplex, but when the germ cells mature in that person they 

 are of two types, namely, with the determiner and without 

 the determiner; and these two types are equally numerous 

 (Fig. 5). This is the phenomenon known as segregation of 

 presence and absence in the germ cells. If both parents 

 are simplex in a character, so that they produce an equal 

 number of germ cells with and without the character then 

 in a large number of offspring, 1 in 4 will have the char- 

 acter duplex; 2 in 4 simplex, and 1 in 4 will not have the 

 character at all (nulliplex). This gives in the offspring of 

 such a pair the famous 3 to 1 ratio, sometimes called the 

 Mendehan ratio. 



Table II 



LAW OP CONDITION OP EYE-CHARACTERS IN CHILDREN BASED ON THE CHARAC- 

 TERS OF THEIR PARENTS 



One parent Other parent Cases Offspring 



brown brown 1, 2, 4 Either all of the children have brown 



eyes, or one fourth have blue eyes 

 brown blue 3, 5 Either all children brown-eyed (though 



simplex) or half blue-eyed 

 blue blue 6 All blue-eyed 



Now the foregoing rules, which we have illustrated by the 

 case of eye-color, hold generally for any positive determiner 

 or its unit character. 



