Explanation Of Gene Action As Related To Physiological

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nearly equally balanced by the humerous other processes which,
themselves, would cause an opposite kind of curvilinearity.
Instead of being surprised at dominance or eplstasls and at the many
cases where herltabillty is rather low, I tend to be astonished that
dominance and eplstasls are not more extreme and that herltabilltles as
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high as i0_o are often found. I suppose natural selection has required
that to be true to a considerable extent, else the specieswould not have
survived. That is, the Mendelian segregation and recombination would have
produced an intolerably large proportion of individuals so defective in
one way or another that they could not survive, if these various growth
processes had combined with each other in extremely irregular ways.
If a species is dependent for survival on having a certain highly _
complex genetic combination, then it can survive only by becoming almost--_
completely homozygous for most of the genes needed in that combination.
This, I am sure, has happenedas concerns differences between major
groupings of the plant or animal world, such as the differences between
Genera, Families and Orders. However, breeders are concerned with
differences such as occur within species and even within breeds. If
these differences are many and if each of them is frequent, then I think
the combinations of their various effects must come somewhere near to
being additive, else the species or breed would have difficulty surviving.
Speculations like these seem to serve no useful purpose, except
that they may help explain how the complicated processes which the genes
initiate or guide can still yield viable individuals most of the time
andhow it happens that the end product can be nearly enough in proportion
to the differences in genes transmitted that we often find
heritabilities of i0 to 20_ or even higher.