I think you misunderstand what ability scores represent.
Humans, to say nothing other creatures, have a wide array of diverse aptitudes. The degree to which any one of these is heritable is difficult to discern, and not well-supported (or well-denied, to be fair) by science. Tons and tons of research have gone into these questions, and have basically managed to come up with a muddled mess that does not clearly confirm any conclusion. A lot of it seems to come down to semantic arguments about how to define and measure whatever particular aptitude you are interested in.
That’s real life. In D&D, we have just six ability scores, rather than the myriad things a person could be naturally suited or unsuited for. A person in D&D is just strong, not strong in some ways but not others. Likewise for each of the other ability scores. These are abstractions, an attempt to fudge all these different aptitudes through a streamlined numerical process that is workable for a game. Abstractions are crucial to having a game that is actually playable, and the choices one makes in the abstractions for a game determine a lot about what sort of game it is.
D&D’s six ability scores, then, is a very high degree of abstraction. Each of the six ability scores is covering infinite different fields of endeavor that one might have natural aptitude for. Remember the state of the art with respect to the heritability of these kinds of traits? A muddled mess, I called it. Lots of semantic arguments about definitions and the appropriateness of various measures. The D&D ability scores have to cover all of those definitions and measures. Those arguments are abstracted away entirely.
Which means that each ability score is rather schizophrenic1 about what it actually represents. It might represent something highly heritable while at the same time representing something that has absolutely nothing to do with genetics and cannot be inherited at all (particularly with the mental ability scores).
On top of this, the typical array of ability scores for NPCs in 3.5 is 11, 11, 11, 10, 10, 10. The thing the average person is best at is only 1 higher than the thing they’re worst at! In other words, ability scores are massively disconnected from our everyday life, where there is great variety. All that variety is stuck in just 1 ability score point. Differences larger than these are extremes rarely if ever found in real life. That means that no matter what we do, we are already dealing with something that is outside the realm of our real-life intuition here.
To go further, even the elite array has a highest score of 15 and lowest of 8. PCs can go to 18, but it’s better to ignore them for this—PCs are not remotely realistic2 and we have absolutely no metric for saying that special something that makes someone a PC is heritable or not. So that 15 represents the height of regular human endeavor,3 and 8 represents the regular nadir. But these scores are exceptional, ability scores that might have someone labeled “freak” and so on. These are only more outside our intuition and understanding of real life, even as limited as that is.
So I would have a child determine his or her scores exactly the same way everyone else does, without any special reference to his or her parents’ ability scores. If the child has similar scores, maybe those were inherited—after all, the ability scores can represent traits that may be genetic. Or maybe they were just the result of being raised by those parents. And if the child is particularly different, maybe that’s because ability scores also can represent things that aren’t inherited at all, and so of course the child would be unique.
For personal experience, I have certainly had many characters across many campaigns who were the children of other characters. Some of them had similar ability scores; others did not. I have never had any difficulty or problem describing this, and have never felt any need to use anything more. Per the above, I actually think that doing so would misrepresent reality more than not.
Used colloquially, to mean having many identities and being pulled in many directions; my psychologist wife might never forgive me. The actual disorder schizophrenia has almost nothing to do with the colloquial usage.
My favorite illustration of this: a 1st-level human barbarian can out-run the world-record sprint speed, but maintain that speed for minutes at a time, and do it while wielding a battleaxe, wearing chain, and carrying 50-odd pounds of gear besides. And when his endurance finally runs out, he isn’t even fatigued—he just can’t run for a minute, and after that he’s totally fine.
I am intentionally not attempting to actualize ability scores by comparing what a given score lets you do (e.g. maximum lift weight for Strength) or is said to be (e.g. the sometimes claim that Int×10 = IQ), since this falls in the trap of trying to treat the ability scores as exactly one thing when, as established, they each cover many, many things.