92 THE THEORY OF THE GENE 



nation of mutations, because, for one reason, it is well 

 known that mutation may occur in the male of Drosophila 

 as well as in the female. In the male of Drosophila there 

 is no crossing-over. 



The Evidence from Multiple Allelomorphs. 



It has been shown in Drosophila, as well as in a few 

 other types (in corn, for example), that more than a 

 single mutation may occur at the same locus. The series 

 of allelomorphs at the locus for white eye in Drosophila 

 is the clearest case of the sort. No less than eleven eye 

 colors, in addition to the red eye of the wild fly, have been 

 recorded. They form a graded series from white to red 

 as follows : white, ecru, tinged, buff, ivory, eosin, apricot, 

 cherry, blood, coral, wine. They have not, however, ap- 

 peared in this sequence, although white was the first 

 mutation observed at this locus. That they have not 

 arisen by the mutation of a series of adjacent genes is 

 clearly shown by their origin and their relation to each 

 other. For example, if the white were due to a mutation 

 from the wild type at one locus and cherry by mutation 

 at an adjacent locus, then when white is crossed to cherry 

 the female offspring should have red eyes, because white 

 would, on this assumption, carry the wild type allelo- 

 morph of cherry and cherry would carry that of white. 

 When white and cherry are crossed they do not give this 

 result, but the daughters have an intermediate eye color. 

 The F^ female gives again white and cherry sons in equal 

 numbers. The same relation holds for all the other alle- 

 lomorphs, any two of which can exist simultaneously in 

 any one female. 



If the presence and absence theory is taken literally 

 there cannot be more than one absence for each gene. The 

 theory in this form is disproven in all cases where the 

 occurrence of multiple allelomorphs is known to have 



