I’m going to steal most of this example, to make a point. The numbers are chosen for convenience.
Measured IQ can be decomposed into two components – genotypic expectation and everything else, which means epistasis, developmental noise, measurement error, environmental influences, etc. We don’t know much about this second component.
Imagine that the narrow-sense heritability of IQ is 0.64 – which means that 64% of the variance in IQ scores can be explained by differences in genotypic expectation, 36% by the second, grab-bag component.
Imagine someone with a +4 SD IQ score (160). On average, that person will have a higher than average value for both genetic and grab-bag components: +3.2 SD for genotypic expectation and +2.4 SD in the miscellaneous component.
Suppose that two people like this marry (a most unusual pairing). Their kids will have an genotypic expectation of +3.2 SD and an average of +0.0 “miscellaneous”, for an IQ averaging +2.56 SD. If two people with that kind of ancestry married, their kids, the next generation, would also have an average IQ of 2.56 SD.
They’re different from the typical kid with an IQ of +2.56 SD. His genotypic expectation will be +2.048 SD, while he’ll have +1.536 SD in the misc component. In other words, with the the typical kid with an IQ of +2.56, a lot of it is luck. Marry two like this and their kids will regress towards the mean (down to +1.6384 SD).
Families will vary. Some parents have high genotypic expectations and hardly any luck: their kids won’t regress much. Others, whose high IQs depended a lot on luck, will have kids that regress a lot.
By looking at the IQ values of close relatives, one can estimate the genotypic expectation component – the “breeding value”. It may also be possible to estimate this by a Visscher-type method, looking at overall genetic similarity with a large set of distant relatives.
In other words, we should be able to estimate the extent to which a high IQ will breed true, particularly easily in large families, but maybe for everybody.
The estimated breeding value is what you want to consider when looking at sperm or egg donors. Or, if you’re into that sort of thing, even in an online dating or matchmaking service.
The ag majors know all this, but anyone majoring in any other discipline is most unlikely to. I don’t think that the average young genomicist knows it.