41 I 



THE AMERICAN NATURALIST [Vol. XLIII 



which can not be as well explained on the basis of a 

 single unit, as a pair of units. 



On this basis the fundamental difference between the 

 three classes of individuals produced by self-fertiliza- 

 tion of a heterozygote may be simply stated thus : There 

 are two classes of homozygotes, usually designated 

 DD and RR, and the heterozygote, usually referred to 

 as DR. The difference between the two kinds of homozy- 

 gotes with respect to any unit-character is, that one— 

 usually the DD— has one pair of allelomorphs or 

 4 'genes" 11 in addition to those possessed by the other 

 kind of homozygote— usually the RR. As the two kinds 

 of homozygotes are often not appropriately called dom- 

 inant and recessive, I will call the one which has the 

 added pair of genes (i. e., the one which has 2m +2 genes) 

 the ''positive" homozygote, and the one which lacks 

 them (i. e., the one with only 2n genes) the "negative" 

 homozygote. If we designate collectively the common 

 features of two parents which differ from each other in 

 a single unit-character by the letters BB, and the differ- 

 entiating genes by the letter A repeated as often as the 

 gene is repeated in each nucleus of the soma or sporo- 

 phyte, the positive homozygote will have the composition 

 AABB, the negative homozygote will be simply BB, and 

 the heterozygote will be ABB; and whatever differences 

 are observable in these three classes of individuals must 

 be due obviously to the presence of none, one, or two 

 "A" genes in each nucleus and to the reactions of these 

 with the underlying factors which have been here collect- 

 ively represented by "BB." 



In order to see the bearing of these assumptions upon 

 questions of dominance we must consider briefly the na- 

 ture of the unit-characters. Eegarding the nature of the 

 genes themselves— the primary character-producing 



internal something or condition upon whose presence an elementary morpho- 

 logical or physiological characteristic depends. The word "gene" has the 

 advantage that it does not assume by its form or derivation any hypothesis 

 as to the ultimate character, origin or behavior of the determining factor. 



