Evolution__3rd_Edition

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Several non-additive genetic factors
are known
Frequency
CHAPTER 9 / Quantitative Genetics 231
The genetic partitioning that we have made so far is incomplete. It applies reasonably
well for one locus: in that case, dominance is the main reason why the genotypic effect
of a parent is not exactly inherited in its offspring. When many loci influence a character,
epistatic interactions between alleles at different loci can occur (Section 8.8, p. 207).
Epistatic interactions are, like dominance effects, not passed on to offspring. They
depend on particular combinations of genes and when the combinations are broken
up (by genetic recombination) the effect disappears; nor are they are non-additive.
An example of an epistatic interaction would be for individuals with the haplotype
A 1 B 2 to show a higher deviation from the population mean than the combined
average deviations of A 1 and B 2 ; the extra deviation is epistatic. Other non-additive
effects can arise because of gene–environment interaction (when the same gene
produces different phenotypes in different environments) and gene–environment
correlation (when particular genes are found more often than random in particular
environments).
A full analysis can take all these effects into account. In a full analysis, just as in the
simple one here, the aim is to isolate the additive effect of a phenotype. The additive
effect is the part of the parental phenotype that is inherited by its offspring.
9.4 Variance of a character is divided into genetic and
environmental effects
0
P
We return now to the frequency distribution of a character. We continue, as usual, to
express effects as deviations from the population mean. If we consider an individual
some distance from the mean, some of its deviation will be environmental, some
genetic. Of the genetic component, some will be additive, some dominance, some
epistatic. These terms have been defined so that they add up to give the exact deviation
of the individual from the mean. Any individual has its particular phenotypic value (P)
because of its particular combination of environmental experiences and the dominance,
additive, and interaction effects in its genotype (Figure 9.5). The different combinations
of E, D, and A in different individuals are the reason why the character shows a continuous
frequency distribution in the population.
x
Character
Figure 9.5
The x-axis of the continuous distribution for a character can be
scaled to have a mean of zero. Consider the individuals (called x)
with phenotype +P. Their phenotypic values are the sum of their
individual combinations of the environmental, additive, and
dominance effects (E + A + D). Individuals with character x can
have any combination of E, A, and D such that E + A + D = x. Any
individual’s deviation from the mean is due to its individual
combination of E, A, and D (as well as other effects, such as
epistatic).