The Study of Human Heredity. 9 



these are the two somatic types. Gametically the germ plasm of the 

 individual may possess alternately germ cells with and without the 

 determiner studied; an individual carrying such a germ plasm is said 

 to be simplex and somatically cannot be easily distinguished from a 

 duplex individual in which every germ cell possesses the determiner 

 in question. The third gametic type is said to be nulliplex in which 

 none of the germ cells possess the determiner in question. There 

 are thus six types of gametic matings in reference to a single char- 

 acter; these types may be expressed as follows: 



Type 1. (D + D) X (D + D)=4DD 



" 2. (D + D) X (D + R)=2DD + 2DR 



" 3. (D + D) X (R + R)=4DR 



" 4. (D + R) X (D + R)=DD + 2DR + RR 



" 5. (D + R) X (R + R)=2DR + 2RR 



" 6. (R + R) X (R + R)=4RR 



D stands for the determiner for the trait studied and R stands for 

 its absence. 



The field worker must understand that research, seeking to un- 

 ravel the laws of inheritance, must work out the gametic nature of 

 each individual studied, hence the necessity of extending the pedigree 

 to all ancestors with collaterals, descendants and consorts of all individ- 

 uals the make-up of whose germ plasm it is desired to understand. 

 For example, by hypothesis, feeble-mindedness is for the most part 

 a recessive trait and the hypothesis must be tested as follows: The 

 field worker finds a person suffering from feeble-mindedness, a 

 descendant of two normal parents — by hypothesis both of these parents 

 are simplex; the field worker must understand that each parent will 

 probably have somewhere in his or her ancestry a feeble-minded 

 person and it is the business of the field worker to make a special 

 search for such person or persons in the pedigree. 



Criticism of an actual pedigree reported by a Held worker. (Plate 

 II. ) This study begins with the epileptic boy III — 7. The principal 

 thing, of course, is to describe accurately all of the brothers and 

 sisters of the affected person, they, being produced by the union of the 

 same two germ plasms, will throw light on the make-up of such germ 

 plasm. The pedigree is to be criticised from this standpoint. More 

 information should be got concerning III — 6, 8 and 9. The field worker 

 at once notes that the mating II — 5 and 6 is the most important one 



