18 HEREDITY IN RELATION TO EUGENICS 



in all of his germ cells; for, were the determiner present 

 anywhere in his organization (including his germ cells) the 

 corresponding character would ordinarily show in his soma. 



In connection with the so-called Mendelian analysis of 

 heredity a nomenclature has grown up which is somewhat 

 different from that here employed. Thus the absent char- 

 acter is often called recessive, the present character domi- 

 nant and the condition in the offspring resulting from a 

 crossing of the two is called heterozygous, which is the 

 equivalent of simplex. It is to be kept in mind that in 

 this work '^absence" does not always imply absolute but 

 only relative absence. Thus the pigmentation of light brown 

 hair is '^ absent " to ^' black/' and ^' tow '' is absent to light 

 brown; but pigment is present in all these grades of hair. 

 To avoid the confusion between relative and absolute ab- 

 sence the terms recessive and dominant are often used to 

 advantage, wherever a series of grades of a character is 

 under consideration. 



These general principles may be rendered clearer by 

 means of a Table of the different sorts of matings of germ 

 cells. And, to focus attention, let us have in mind a con- 

 crete example; that of pigment of the iris of the human 

 eye. In the following table P stands for the determiner 

 of brown pigment and p for its absence. Six sorts of unions 

 are possible. See also Plate I, frontispiece. 



Table I 

 Laws of inheritance of characters based on conditions of the deter- 

 miners IN THE PARENTAL GERM PLASMS 



determiners 

 Case One -parent Other parent Offspring Characteristics of offspring 



1 PP PP PP,PP All with pigmented iris 



(brown-eyed) 



2^ PP Pp PP, Pp All pigmented, but half sim- 



plex 



