At this point in time we don't have a handle on complex polygenic traits, which are actually the traits of the most importance-- like IQ, diabetes risk, schizophrenia risk, autism risk, obesity risk, etc.-- but with larger and larger GWAS studies we're going to know a lot more in 5 years, 10 years, etc.
Regarding complex genetic interactions: we can quantify how much of the variance is additive versus non-additive. The former is termed narrow-sense heritability and is the portion that contributes to parent-offspring resemblance. The reason this distinction is important is because the latter is the unpredictable part of genetic engineering--the non-linear interaction effects that people are afraid of, the strange effects that might result from certain combination of genes, etc. The former (additive variance) is the variance that's exploited in animal breeding to produce higher breeding-value cows, chickens, etc. in a mostly predictable fashion.
From this coursera course on behavioral genetics
>>So, the difference between MZ and DZ twin correlations, corresponds to half the additive genetic effects. I told you that there is a very general pattern in the behavioral genetic literature. MZs are more similar than DZs. The MZ correlation is greater than the DZ correlation. By putting in this biometric formulation, what we can actually do is begin to quantify. That impression that we get just by looking at the correlations. And this is telling us how to quantify it. The difference corresponds to half the additive genetic effect. Therefore if we double the difference. We get an estimate of the additive genetic variance.
>>The added genetic variance is estimated as taking the difference in, in the two correlation and doubling it. And for height, the heritability estimate here is roughly 70%. Again, this is going to vary. It's a proportion between zero and a hundred. Her IQ is a little bit less. It's about 60%. What do, how do we interpret that? We say that, about, 60% of individual differences in IQ, appear to be associated with genetic differences among individuals, additive genetic differences. It turns out, just coincidentally, in this particular dataset, the estimate of shared environmental effects is just 20% in both cases. Non-shared environmental effects, 8 about 10% here, 20% there. And note that these do add up to 100% as they should. So, what the biometricians are giving us here, is a way to begin to quantify these underlying sources of individual differences. And, and interpret I'll just focus on IQ here, because it's the a behavioral trait, one that we'll be interested in, in this course. What, this would say is that, it looks like roughly 60% of the variance in IQ is associated with genetic factors.
From a 2017 GWAS study with ~78k people:
>>Our calculations show that the current results explain up to 4.8% of the variance in intelligence and that on average across the four samples there is a 1.9-fold increase in explained variance in comparison to the most recent GWAS on intelligence6.
If we end up with, say, enough knowledge to predict 1/3 of the additive genetic variance in IQ, and we combine that with Preimplantation Genetic Diagnosis, depending on the # of fertilized zygotes available, success rates of implanation, $ willing to spend, etc. you could very feasibly do some very interesting things.
TL;DR
I agree we don't know much about those polygenic traits quite yet but I think we will know a whole lot more in 5 or 10 years, and people aren't quite ready for it...
Reccomended reading:
-
4th law of Behavioral Genetics
-
3 laws of Behavioral Genetics and what they mean