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[TURALIST [Vol. XLHI 



may mean that already the presence of the one unit A of 

 the heterozygote is sufficient to result in the maximum 

 reaction, in which case the doubled factor AA of the posi- 

 tive homozygote can do no more. When, on the other 

 hand, one unit A is not sufficient to produce a maximum 

 reaction with the other factors present, the AA of the 

 homozygote produces the corresponding character in 

 greater intensity, and the heterozygote will be intermedi- 

 ate between the two homozygous parents. Both of these 

 conditions are frequently realized. 



The case I have wished to deal with specifically is that 

 in which the heterozygote— the ABB individual— does 

 not differ in external aspect from the negative homozy- 

 gote, BB, so that the ratio becomes 1 : 3 instead of 3 : 1, 

 this is the situation in which the absence of a character 

 is dominant over its presence. In such a case the char- 

 acter determined by A is latent in the heterozygote. To 

 show that this situation is possible it need only be pointed 

 out that in a number of familiar instances a precipitate 

 is formed or some other visible reaction takes place only 

 in the presence of a certain excess of one of the reagents. 

 It is perfectly clear that in any such case, one may add 

 nearly enough of the reagent which is required to be in 

 excess, and no apparent reaction will take place, but if 

 the quantity of this reagent be doubled the characteristic 

 reaction will occur. Now this is just what I conceive 

 may take place in certain crosses. In the heterozygote 

 where the chemical unit A (of whatever nature) occurs 

 but once in each nucleus, no reaction becomes apparent, 

 but in the pure-bred forms bearing the unit A in double 

 quantity, i. e., A A, the specific character (or reaction) 

 produced by this unit appears. The heterozygotes will 

 then be indistinguishable from the negative homozygote, 

 and in the offspring of two heterozygotes bred together 

 there will be among every four individuals on the average 

 three which have the character absent and one which has 

 it present, or in other words "absence will be dominant 

 over presence." 



