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Evolution__3rd_Edition

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..<br />

Several non-additive genetic factors<br />

are known<br />

Frequency<br />

CHAPTER 9 / Quantitative Genetics 231<br />

The genetic partitioning that we have made so far is incomplete. It applies reasonably<br />

well for one locus: in that case, dominance is the main reason why the genotypic effect<br />

of a parent is not exactly inherited in its offspring. When many loci influence a character,<br />

epistatic interactions between alleles at different loci can occur (Section 8.8, p. 207).<br />

Epistatic interactions are, like dominance effects, not passed on to offspring. They<br />

depend on particular combinations of genes and when the combinations are broken<br />

up (by genetic recombination) the effect disappears; nor are they are non-additive.<br />

An example of an epistatic interaction would be for individuals with the haplotype<br />

A 1 B 2 to show a higher deviation from the population mean than the combined<br />

average deviations of A 1 and B 2 ; the extra deviation is epistatic. Other non-additive<br />

effects can arise because of gene–environment interaction (when the same gene<br />

produces different phenotypes in different environments) and gene–environment<br />

correlation (when particular genes are found more often than random in particular<br />

environments).<br />

A full analysis can take all these effects into account. In a full analysis, just as in the<br />

simple one here, the aim is to isolate the additive effect of a phenotype. The additive<br />

effect is the part of the parental phenotype that is inherited by its offspring.<br />

9.4 Variance of a character is divided into genetic and<br />

environmental effects<br />

0<br />

P<br />

We return now to the frequency distribution of a character. We continue, as usual, to<br />

express effects as deviations from the population mean. If we consider an individual<br />

some distance from the mean, some of its deviation will be environmental, some<br />

genetic. Of the genetic component, some will be additive, some dominance, some<br />

epistatic. These terms have been defined so that they add up to give the exact deviation<br />

of the individual from the mean. Any individual has its particular phenotypic value (P)<br />

because of its particular combination of environmental experiences and the dominance,<br />

additive, and interaction effects in its genotype (Figure 9.5). The different combinations<br />

of E, D, and A in different individuals are the reason why the character shows a continuous<br />

frequency distribution in the population.<br />

x<br />

Character<br />

Figure 9.5<br />

The x-axis of the continuous distribution for a character can be<br />

scaled to have a mean of zero. Consider the individuals (called x)<br />

with phenotype +P. Their phenotypic values are the sum of their<br />

individual combinations of the environmental, additive, and<br />

dominance effects (E + A + D). Individuals with character x can<br />

have any combination of E, A, and D such that E + A + D = x. Any<br />

individual’s deviation from the mean is due to its individual<br />

combination of E, A, and D (as well as other effects, such as<br />

epistatic).

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