[ExI] IQ and beauty

[Rafal Smigrodzki]

On Mon, Sep 14, 2015 at 1:19 PM, PJ Manney <pjmanney at gmail.com> wrote:
>
>
> When it comes to intelligence, it's the mother's genes that count in boys.
> They've only got one X chromosome which expresses intelligence. That's why
> you sometimes see the sons of the wealthy and powerful are as dumb as a
> sack of hammers, if dear old dad only married for beauty.
>

### You may be referring to a hypothesis outlined in  Lancet.
<http://www-ncbi-nlm-nih-gov.proxy.library.vcu.edu/pubmed/8667929#> 1996
Jun 29;347(9018):1814-5, "Intelligence and the X chromosome",  by Gillian
Turner (or do you have other references?)

Indeed, children inherit a slightly larger amount of DNA from their mothers
(mitochondrial DNA, and in the case of males, the X chromosome), and this
could lead to a higher correlation between IQ of mothers and children,
especially sons. However, the difference in the amount of inherited nuclear
DNA is very small (100 million basepairs, so about 1.5% of the diploid
genome), and it is unlikely to account for a measurable fraction of
heritability, unless there is preferential accumulation of IQ-influencing
alleles on the X chromosome. Turner claims there is, based on the
assumption of about 325 recessive autosomal alleles responsible for mental
retardation and the detection of at least 154 X-linked ones (which is
obviously much more than 1.5%). However, that article really is rather
antediluvian, published in the 7th year of the B.S.H.G era (Before
Sequencing of Human Genome) and I have no idea how they came up with the
strangely low estimate of 325 autosomal recessive IQ alleles. In the 19
years since its publication it became quite obvious that there thousands of
genetic variations that influence intelligence. Also, if the X chromosome
preferentially accumulated IQ genes, we would expect a much higher
correlation between IQ of mothers and sons than between fathers and sons.
Turner fails to present any data to this effect - do you have any
references that would rectify this situation?

In the absence of such correlational data, the hypothesis of preferential
accumulation of IQ-influencing genes on the X chromosome would have to be
rejected.

Also, Turner only points to data on the inheritance of mental
*retardation*, not normal intelligence. The case for the hypothesis as
applied to normal or superior IQ (as popularized in newspapers of the time)
is therefore even more flimsy.

Rafał
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