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Additive Effects vs Epistatic Effects

Genetics as it applies to evolution, molecular biology, and medical aspects.

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Additive Effects vs Epistatic Effects

Postby ReMuSoMeGa » Tue Jan 24, 2012 10:45 pm

I'm having a hard time understanding the finer points (and consequences) of these two definitions.
Heres what I know:
Additive effects are when genes _at_ multiple loci can independently contribute to a phenotype.

Epistatic - I'm finding several different definitions. One of them says this happens when gene supreses the effect of another gene at a different locus. Although my understanding of this, was simply: A complicated, non-additive interaction between multiple loci which will alter a phenotype (in no real cut & dry way). Is my second 'definition' more correct?

So, in the case of complete dominance (on the same loci) can we call this an epistatic effect?
What about co-dominance/incomplete dominance? Is this also epistatic? IE. the 3 genotypes: AA = Black Fur, aa = White Fur, Aa = Grey
In that previous example, doesn't that seem somewhat additive though?

Finally, my last question. My professor briefly mentioned that only additive effects are inheritable. Why is this?
If a Loci independently contributes to a phenotype, wouldnt the formation of gametes disrupt this? We are passing only 1 allele of each loci in question to the offspring. Does this mean that that the effects of the alleles at the SAME loci must ALSO be additive?

Now what about in the case where we have 3 loci that independently contribute to phenotype A. However, the relationships between the alleles at each respective loci are epistatic.

What do we call this? Is this still inheritable?

Thanks for reading!
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Postby JackBean » Wed Jan 25, 2012 3:22 pm

the additive and epistatic effects are relevant only for DIFFERENT loci!

Cis or trans? That's what matters.
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