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From this article, second paragraph of the second page

A classic theoretical result is that the mean of a character controlled by a single locus $i$ with two alleles $A_{i1}$ and $A_{i2}$ is only affected by the value of $f$ if there is some degree of dominance.

, where $f$ is the Wright's inbreeding coefficient.

The authors then add:

Assume that the trait is scaled such that the values for the genotypes $A_{i1}A_{i1}$, $A_{i1}A_{i2}$, and $A_{i2}A_{i2}$ are $-a_i$, $d_i$, and $a_i$ respectively. The quantities $a_i$ and $d_i$ measure the effect of the locus on the character and the degree of dominance of the locus, respectively. With two alleles, there is a linear decline in the mean of a trait with increasing $f$ either with overdominance ($d_i$ > $a_i$) or if the allele associated with an increased value of the trait is dominant or partially dominant ($a_i$ ≥ $d_i$ > 0). With additive joint effects of different loci on the trait, this conclusion can be extended to a polygenic character; inbreeding decline occurs if the average value of $d_i$ over all $i$ is positive.

I don't understand why "[..] there is a linear decline in the mean of a trait with increasing $f$ either with overdominance ($d_i$ > $a_i$) or if the allele associated with an increased value of the trait is dominant or partially dominant ($a_i$ ≥ $d_i$ > 0)"

Can you please explain why the mean of a character controlled by a single locus is only affected by the value of $f$ if there is some degree of dominance.

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man i should be able to answer this. i didnt take that class though. I could write a doctor who knows the answer but i doubt he would respond. –  caseyr547 May 14 at 6:09

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