CHAPTER X 
THE FACTORIAL HYPOTHESIS 
In Mendelian heredity the word “factor” is used 
for something which segregates in the germ cells, 
and which is somehow connected with particular 
effects on the organism that contains it. For exam- 
ple, if a fly (°) with red eyes is crossed to a fly (¢) 
with white eyes, there will be in F, three reds to one 
white, and this ratio can be explained by the assump- 
tion that in the F, hybrid something for red eyes 
has separated from something for white eyes. 
We may express these factorial relations in another 
way by saying that a germ cell that produces white 
eyes differs from a germ cell that produces red eyes 
by one factor-difference. We think of this difference 
as having arisen through a factor in the red-eyed wild 
fly mutating to a factor for white. 
Mendelian heredity has taught us that the germ 
cells must contain many factors that affect the same 
character. Red eye color in Drosophila, for exam- 
ple, must be due to a large number of factors, for as 
many as 25 mutations for eye color at different loci 
have already come to light. Each produced a specific 
effect on eye color; it is more than probable that in 
the wild fly all or many of the normal allelomorphs at 
these loci have something to do with red eye color. 
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