ADE and submodels

> I noticed in the ADE code that only AE and E models are considered
> why isn't the DE model a possible model to consider?

It's considered unlikely that all genes for a behavior will be non-additive, with no additive effects.

One can imagine such a trait – simplistically coded eye colour, for instance – which consists of a single Mendelizing dominant allele + unshared noise (DE).

But for polygenic traits, it's hard to generate data from plausible sets of gene alleles where the trait doesn't show an additive signal.

So the reasonable model is ADE, with A droppable only due to lack of power.

Often people will drop D leaving an AE model. This, no doubt also commonly reflects lack of power also, with the null hypothesis of strictly zero D being unlikely also for traits that have seen any selection.

The choice of including D OR C is, of course, invidious.

Recently people (Zuk et al.) have suggested that reduced DZ similarity due to D (perhaps gene-gene interaction) combined with increased DZ similarity due to C might make AE twins reared together models fit better than they should (if, say, we had adoption data in the mix).

Does anyone have the (Purcell? Neale?) ref on where unmodelled variance goes in twin studies - that's worth citing here.

It would be interesting if your DE model fit better than AE. I would mount a statistical argument against that on the principle of marginality: the principle of not including interactions (which D is) in the absence of main effects to ensure that main effect variance is not aliased into the interaction term. i.e., dropping the degree of freedom might work in terms of AIC, but is fraught in causal terms.

Just for completeness, this question typically arises from a researcher who wants to test not a DE model, but a DCE model (i.e., to retain shared environment and drop additive genetic effects).

I think (others might chime in) that the logic arguing against this is that an inability to identify the model we should use (ADCE) is not justification for using a model (by the argument above from additivity) that we think is implausible (DE or DCE).

See also this paper from the always-worth-reading Neil Risch for an explanation of the related expectation that we should not (commonly) expect mechanistic interactions in the absence of main effects (here in the context of a critique of GxE interactions for depression.

http://www.ncbi.nlm.nih.gov/pubmed/19531786