CHAPTER IX
'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 (0")
with white eyes, there will be in F2 three reds to one
white, and this ratio can be explainedby theassump-
tion that in the Fi 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 f actor 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 fiy all or many of the normal allelomorphs at
these loci have something to do with red eye color.

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