REACTIONS TO LIGHT AND GRAVITY IN DROSOPHILA 89 
their fathers, but do not show the character, while sons inherit 
the factor from their mothers and do show the character.* 
The independence of the light reaction from the color as such 
may be realized from the following facts. Tan, like most other 
characters, varies about a mode. Furthermore, this particular 
character is so delicate, that at the extreme of variation toward 
the normal color it is quite impossible to distinguish the indi- 
vidual possessing it from wild stock. This being the case, it 
sometimes happened that an insect whose genetic constitution was 
really tan would be accidentally mixed with flies which were 
supposed to be normal and vice versa. For example, one case 
occurred of a male fly which was apparently tan. Its light test, 
however, proved to be that of a normal. To test it, therefore, 
it was bred to a tan female. If the father were really tan, as it 
appeared, its daughters would all be tan. If, however, it were 
really normal as its light reaction indicated, then all its daugh- 
ters would be normal. The latter turned out to be the case. 
The daughters were normal both in appearance and in light re- 
action. Later a case arose where several normal appearing fe- 
males reacted as though tan. They were bred to a normal 
male. If they had been normal then all of the offspring should 
have been normal; if they were heterozygous then all the females 
would have been normal but half the males tan. What hap- 
pened, however, was that all the females were normal, since tan 
is recessive, but all the males were tan. This proved that the 
females were really all homozygous tans as they had indicated 
by their light reaction, though their appearance had belied the 
fact. Thus it developed that light reaction was a surer test for 
the character of tan than was the color itself. 
It remains merely to give a table showing the records of 
three groups of twenty tan flies each. They were tested ac- 
cording to the most recent system of light and gravity tests 
(table 21). 
Table 21 offers conclusive evidence that the failure of tan flies 
to respond to light is not due to any general inactivity. This is 
>For a full discussion of Mendelism in Drosophila, see the Mechanism of 
Mendelian Heredity by Morgan, Sturtevant, Muller and Bridges. 
