Explanation Of Gene Action As Related To Physiological

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strains were very large. If_ however, we can find a weak strain which is two
or three units below the threshold and can progeny test the individuals
of the other strains on this weak tester strain, then the progeny averages
will show clearly which strain contained the larger margin of safety.
Breeding for disease resistance may well go in this direction wherever
such a weak strain can be maintained and there is no more economical way
to combat disease.
Breeders of corn have used this method freely in
breeding for stronger stalks and to reduce the percentage of ears dropped
in the field before harvest. The basic reason it is practical in these
threshold cases to use weak or defective strains as special tester
stocks is that we can not measure how far the individual is above the
threshold of breakdown.
Wherever we can find a way to measure the factor
of safety or margin of resistance in each individual, then these threshold
characters can be
treated the same as any character which can be
measured on a continuous scale.
If we know that other kinds of epistatic factor actions are important,
we will, in general, rely more on linebreeding than if we know they
do not exist.
We will do much making of families and subsequent selecting
between those families on the average performance of the family. We
will rely more on the family average than on the individual the surer
we are that we are working with such an epistatic situation.
We will
intercross the very best families and make new lines out of those crosses
which are best.
Incidentally, this is just the exact opposite from what
we would do if we knew that overdominance were important.
That is, if we
know that overdominance is important and we find two families which cross
extremely well, then we know that we ought not try to make new families
from that cross.
Instead, we would make new families out of crosses
among families which all cross well with family Q, for example.
Then we
would test these new families in crosses with family Q.
Similarly, if we