No. 518] 



VARIATION 



zygote in the same manner that they do when combined, 

 the F 2 generation will appear like the dominant parent 

 unless a very large number of progeny are under observa- 

 tion and pure recessives are obtained. This may be an 

 explanation of the results obtained by Millardet; it is cer- 

 tainly as probable as the hypothesis of the non-formation 

 of homozygotes. Ordinarily, however, there is not per- 

 fect dominance, and variation due to heterozygosis com- 

 bined with fluctuating variation makes it almost impos- 

 sible to classify the individuals except by breeding. The 

 two yellows in the endosperm of maize is an example of 

 how few characters are necessary to make classification 

 difficult. First, there is a small amount of fluctuation in 

 different ears due to varying light conditions owing to 

 differences in thickness of the husk; second, all the classes 

 having different gametic formula? differ in the intensity 

 of their yellow in the following order, Y^Y^Y^Y*, 

 Y,y,Y 2 Y 2 or YJ x Y 2 y 2 , Y,Y U Y 2 Y 2 , Y x y x , Y 2 y 2 , y,y 2 . As 

 dominance becomes less and less evident, the Mendelian 

 classes vary more and more from the formula (3 + l) n , 

 and approach the normal curve, with a regular gradation 

 of individuals on each side of the mode. When there is 

 no dominance and open fertilization, a state is reached 

 in which the curve of variation simulates the fluctuation 

 curve, with the difference that the gradations are herit- 

 able. 



One other important feature of this class of genetic 

 facts must be considered. If units AyA 2 A^a 4 meet units 

 a^a^A^ in the F 2 generation there will be one pure re- 



anlp^aTent paradox. Two individuals are crossed, both 

 seemingly pure for presence of the same character, yet 

 one individual out of 256 is a pure recessive. "When we 

 consider the rarity with which pure dominants or pure 

 recessives (for all characters) are obtained when there are 

 more than three factors, we can hardly avoid the suspicion 



