Explanation Of Gene Action As Related To Physiological

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! SUB-DIVISION OR CLASSIFICATION OF THE RESULTS OF GENE ACTION
You are all moderately familiar with outward effects being subdivided
into additive, dominance or epistatic. This classification is
only an operational one. It does not mean that we know exactly what
each gene does or could write the chemical formulas of the reactions
which it causes or controls. We classify the gene effects in this way
because they respond differently to different breeding methods, The
additive effects we can improve by mass selection, reinforced perhaps
with attention to relatives, to sibs, and to progeny tests. The nonadditive
effects are truly hereditary in that they are caused by
differences in the genes which different individuals have; yet they
contribute irregularly or not aZ all to the resemblances between
relatives.
These nonaddltive effects are split into two groups; ..... the
dominance deviations and the epistatlc deviations, .... because the
former can never be transmitted in a single gamete and, therefore,
contribute not a_ all to the likeness Of any relatives except those
who are related through at least two lines of descent which do not
anywhere go through the same gamete. Many of the eplstatic effects do
contribute something (usually not much unless we are concerned with
highlY inbred lines) to the resemblance of individuals related through
only one line of descent.
The additive effect of a gene is the average effect of substituting
it for its allele without any other change. If the substitution actually
has an effect larger than average in some individuals, it must
necessarily have an effect smaller than average in others. These differenceslbetween
the actual effects of the gene, in all sorts of
individuals which interest us, and its average effects are the nonadditive