Ordinarily one of these produces a more notable contribution to the
end-product than do the others; but there are cases where this statement
does not appear to hold good. This leads to the conception of _multiple
factors_.
In crossing a wheat with brown chaff and one with white chaff, H.
Nilsson-Ehle (1909) expected in the second hybrid generation to secure a
ratio of 3 brown to 1 white. As a fact, he got 1410 brown and 94 white,
a ratio of 15:1. He interpreted this as meaning that the brown color in
this particular variety was due not to one factor, but to two, which
were equivalent to each other, and either one of which would produce the
same result alone as would the two acting together. In further crossing
red wheat with white, he secured ratios which led him to believe that
the red was produced by three independent factors, any one of which
would produce red either alone or with the other two. A. and G. Howard
later corroborated this work,[48] but showed that the three factors were
not identical: they are qualitatively slightly different, although so
closely similar that the three reds look alike at first sight. E. M.
East has obtained evidence from maize and G. H. Shull from
shepherd's-purse, which bears out the multiple factor hypothesis.
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