... So… most genetic load in humans is made up of many, many mutations that each have fairly small effects. A smaller fraction of the genetic load consists of mutations with big effects on fitness.
... One important point is that a single highly deleterious mutation has a good chance of pushing the whole organism in some odd direction in phenotype space. In other words, the same mutation that drops your IQ, or damages your heart, may also make you look funny. At lower IQs, more and more kids are considered to suffer from ‘organic’ retardation. On the other hand, a higher-than-average number of small-effect mutations should also interfere with really complex systems such as the brain (and reduce IQ), but because of the law of large numbers, wouldn’t tend to have any particular direction in phenotype space. As far as I can tell, an extra-large dose of small-effect mutations, which we will henceforth call genetic noise, would not make you funny-looking.
Individuals can vary in the amount of genetic noise they carry, and populations can as well, depending on the relative intensity of selection and on the mutation rate, which might also differ. For example, although having an unusually old father does not much affect the amount of genetic noise an individual carries, a culture in which fathers were typically 55 would undoubtedly accumulate an unusually high amount of genetic noise, over a couple of millennia.
If a kid’s parents have a higher-than-average amount of genetic noise, on average the kid will as well. This sure looks like what we usually call non-organic or familial retardation.
Most of the within-population variation in IQ looks to be familial rather than organic. If I’m right, this means that most IQ variation – what we might call the normal range – is caused by differences in the number of slightly deleterious mutations. None of them would show up in a QTL search, because all are rare. And that is where we stand thus far: no intelligence QTLs have been found – although you never know what you’ll see in the next population. On the other hand, shared chromosomal segments would mostly contain the same slightly deleterious mutations, and so IQ should correlate with genetic similarity, which is what Visscher has found.
Many great scientists and mathematicians have likely had relatively low levels of genetic noise combined with some fairly deleterious de novo mutations; with the net effect of a powerful mental engine strangely focused on some particular topic not directly related to fitness. Low noise, high weirdness. Math, not sheilas. One might look for advanced paternal age in such cases.