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.—F actors 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 
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