134 GENETICS IN RELATION TO AGRICULTURE 



the normal allelomorph for this factor. Plants possessing the recessive 

 factor may be recognized in the seedling stages by a peculiar coloration 

 of the edges of the leaves and even better by the characteristic epidermis 

 of the leaf blades. 



Manifold effects of factors are probably very common but very little 

 definite work has been reported along this line. Morgan, however, has 

 called attention to some cases in Drosophila. Thus there is a factor for 

 club wings, and in strains of this type flies appear the wing pads of which 

 fail to unfold after emergence. But this character is not constant, in 

 fact about 80 per cent, of the flies in a pure strain have normal wings. 

 Subsequent study, however, has shown that in such stocks the absence 

 of spines on the side of the thorax is a constant differential test. These 

 differences are shown in the accompanying figure (59). By employing the 

 absence of spines as the differential test it is possible to classify mixed 

 populations of "normal" and "club" flies accurately without paying any 

 attention to wing characters. 



The Variability of Factor Expressions. Factors also vary in the effects 

 which they produce. We have pointed out that in pure strains of club- 

 winged Drosophila (Fig. 59) only about 20 per cent, of the flies exhibit the 

 unfolded wing pad characteristic of the club mutation. On the other 

 hand, the absence of spines on the side of the thorax determined by the 

 same factor appears to be an invariable characteristic of the club-winged 

 flies. 



Sometimes this variability in factor expression may be traced to a defi- 

 nite environmental condition. This is certainly true of the red Primula 

 which produces red flowers under ordinary temperature conditions, 

 but which when placed under abnormally high temperatures produces 

 white flowers. The production of chlorophyll in some strains of corn, 

 likewise, depends on generally favorable environmental conditions. 

 This has been demonstrated by Miles for the yellow-green type of 

 chlorophyll reduction. Plants heterozygous for this factor produce 

 grains three-fourths of which produce fully green plants on germina- 

 tion, but the other one-fourth produce pale yellowish seedlings with a 

 tinge of green. The yellowish seedlings die under ordinary conditions, 

 but in particularly favorable surroundings they continue to live and soon 

 develop the normal chlorophyll coloration. If self-fertilized, they produce 

 only yellowish plants which must again be given very favorable condi- 

 tions for the production of the normal green leaf color. 



In Drosophila a number of environmental relations have been de- 

 scribed. Thus Morgan has studied in considerable detail the influence 

 of environment on the development of abnormal abdomen. Flies with 

 the dominant factor for abnormal abdomen should all exhibit the char- 

 acteristic type of deformed abdomen shown in Fig. 60; but this is not the 



