In twin genetic models, why is it that it’s not possible to estimate the effect of dominant genetic factors (D) without additive genetics factors (A)?
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Because genes interact all the time, sometimes regardless of whether an allele is dominant or recessive. So it’s borderline impossible to say “gene A has a dominant allele 1 and that has effect X” because effect X may very well be partially influenced by gene B, C, or D.
In this example, you could study the effect of A by comparing the effects of A with all possible combinations of B, C and D. But in real life, with millions of genes, and many that we don’t even really know about, we can’t say for sure that gene A has a dominant allele 1 that always leads to effect X because it may be that effect X is actually caused by the additive effect of gene A and gene Z.
For example, you can’t technically study the effect of say a gene that determines bone shape, without the gene that determines whether bones are formed at all. Can’t study bones if they aren’t there. But you can’t exclude the possibility of both genes interacting and maybe technically the bone shape gene would make a butterfly shaped bone if it weren’t for the additive effect of the bone presence gene. We just can’t possibly know
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