Explanation Of Gene Action As Related To Physiological

only when both are present. Inhibitory genes prevent other genes from
doing what they would do if the inhibitory genes were absent. Multiplicative
effects are common in differences in growth, especially when
growth is measured as weight or volume. If a gene increases weight 5%
it makes a bigger increase in a bird which weighs 4 lbs. than in one
which weighs 2 lbs. In this way the actual effect of the gene varies,
according to the other genes present. Very often in nature and in animal
breeding the intermediate dimension or quality of the character is
preferred over either extreme. If the character is affected by only one
gene, this becomes overdominance. But if it is affected by many genes,
the intermediates are only a tiny bit more heterozygous than the average
individual in the population and the genetic situation and problem is
very different from that of overdominance. The threshold type of epistasis
is likewise very common. It is usual for characteristics which
can be measured only in two levels, such as mortality, or the breaking
strength of bones. Tkethresholds need not be absolutely sharp. When
they are gradual, the individual differences may be measured on a nearly
continuous scale through a zone just below to just above the threshold
but, when we get far below or far above, further genetic changes make
little or no difference in the outward effect, This leads to a "factor
of safety" type of epistasis which is particularly interesting because
it is the commonest kind in which conducting the progeny test on very
weak or very susceptible special stocks may be worth while.
PLEIOTROPY
The foregoing is phrased as if a gene had only one effect. But
already we know many cases in which a single gene clearly affects several
characters. The usual rule is that the more we study a gene, the more
effects we learn it has. I am inclined to conjecture that nearly all