[50]
It does not fall wholly into the class of blending inheritance, for it
does segregate to a considerable extent, yet some of the factors may
show blending. Much more psychological analysis must be done before the
question of the inheritance of feeble-mindedness can be considered
solved. But at present one can say with confidence of this, as of other
mental traits, that like tends to produce like; that low grades of
mentality usually come from an ancestry of low mentality, and that
bright children are usually produced in a stock that is marked by
intelligence.
Most mental traits are even more complex in appearance than
feeble-mindedness. None has yet been proved to be due to a single
germinal difference, and it is possible that none will ever be so
demonstrated.
[Illustration: FIG. 24.--The twins whose finger-prints are
shown in Fig. 25.]
Intensive genetic research in lower animals and plants has shown that a
visible character may be due to
1. Independent multiple factors in the germ-plasm, as in the case of
wheat mentioned a few pages back.
2. Multiple allelomorphs, that is, a series of different grades of a
single factor.
3. One distinct Mendelian factor (or several such factors), with
modifying factors which may cause either (a) intensification, (b)
inhibition, or (c) dilution.
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