No. 576] VARIATION IN DROSOPHIL A 713 



Further analysis has shown that many of these early 

 cases are readily interpreted on Mendelian principles, 

 while for other exceptions, like that of Hieracium, for 

 instance, the trne explanation has been found without in 

 any way coming into conflict with Mendelism. 



The masking of a Mendelian ratio may be effected in 

 many ways, and some of the most important of the recent 

 work in genetics has dealt with this problem. Among 

 the conditions so far brought to light may be mentioned 

 the following : 



(a) Multiple Factors.— Recent papers by MacDowell 

 (1914) and Shull (1914) have discussed at length the 

 literature and history of this subject. In brief, the work 

 that has been done shows that in both animals and plants 

 the production of certain characters is brought about 

 through the action of two or more independently Men- 

 delizing pairs of genes that have similar effects on the 

 developing organism. If the effect of these genes is 

 cumulative, so that the character is more or less produced 

 according to the number of dominant genes present, the 

 type of inheritance known as blended inheritance is 

 produced. If the effect is not cumulative, the recessive 

 character does not appear with the frequency of 1 : 3, but 

 with the frequency of 1 : 15, 1 : 63, etc., according to the 

 number of pairs of genes concerned. 



(b) The Effect of the Environment. — A typical case 

 of this sort is reported by Baur (1912). In crossing a 

 dark red to a red strain of Antirrhinum, a complete series 

 between the red and the dark red appeared in the F 2 gen- 

 eration; the effect of light on the plants was such that 

 plants that had developed in a bright light had a darker 

 color than those that had developed in a less intense light. 

 The analysis of the F 3 generation, however, proved con- 

 clusivelv that one fourth of the F 2 plants had been homo- 

 zygous dark reds, one fourth had been homozygous red, 

 and two fourths had been heterozygotes. Morgan (1912a) 

 has described a case in Drosophila in which moisture 

 conditions in the bottle in which the flies are developing 

 determine to a certain extent whether or not certain 



