THE INHERITANCE OF PRODUCTIVITY IN FARM LIVE STOCK
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For instance, in bacon swine too thick a back is as undesirable as a back too thin.
Hence, although the genes affecting thickness of back might perhaps act additively
in terms of linear measurements to make the back thicker or thinner, yet they would
not act additively when measured on the scale of how much more or less desirable
they make the bacon carcass. That is illustrated schematically in the accompany-
ing diagram. Another example, outwardly different but the same in principle, may
be furnished by dressing per cent in swine. Other things being equal, the higher
the dressing per cent the more desirable the pig from the butcher's view point.
From the grower's standpoint there must be some limit to this. The pig must have
a certain weight and volume of digestive and other vital organs if it is to be healthy
and thrifty. Hence, considering everything, the ideal pig in this respect will be
something of a compromise between two partially irreconcilable ideas, having a
dressing per cent somewhat lower than the butcher wishes but somewhat higher
than the grower would wish if he could afford to disregard the butcher's desires.
It is characteristic of such cases that the individual genes cannot be rated as either
good or bad, since the directions of their effects are so dependent upon the combina-
tion of other genes present. Hence selection is for or against gene combinations
as such, rather than for or against the constituent genes of those combinations.
On the other hand, the mechanism of inheritance is such that the genes and not the
gene combinations are the units of inheritance.
The problem presented by such variance can be solved by the use of inbreed-
ing to differentiate the stock into genetically distinct strains or families, with rare
outcrosses between such families to restore the variance and make new combina-
tions possible. Such a differentiation into lines should permit inter-family selec-
tion, which would be more effective than individual selection. However, the
proper balance to be maintained between the intensity of inbreeding, frequency of
outcrossing, and intensity of individual and family selections, in order to promote
the maximum rate of progress, offers a whole series of problems, the complete
solution of which may well vary from case to case and require continued research
for years to come on a scale not yet approached in any country.
In summary, then, we are agreed that inheritance is complex, and that each
trait must be studied for itself from several points of view. We are also agreed
that the first practical step in breeding is selection under an environment which
will permit the genetic differences between individuals to manifest themselves as
definitely as possible. We are agreed that progeny tests and, at least, a little
initial attention to pedigree are quite helpful. We are not agreed as to the relative
amounts of attention that should be given to individual performance, pedigree,
and progeny test. We are agreed that it is desirable (as Mr. Smith especially
has emphasized) to control environment as much as possible, so as not to be misled
by it when making selections, but we differ in our optimism as to how complete
control can be in actual practice. We are not entirely agreed as to the gains that
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