On Sun, Jul 17, 2016 at 08:34:56AM +0200, Abu Dhabi wrote:
> > Contact, trade, and sex with extraterrestrial human races occurred
> > long before any Terran colonies were established.
>
> But did that lead to substantial admixture in the Terran population?

Yes.

> I'm very skeptical that even in the 4000 or so years since contact,
> the amount of extraterrestrial genes entering Terra was substantial
> enough that most inhabitants of the Terran system would have some.

Humans have on the order of a hundred thousand genes, depending upon
how you count.  So if more than one part in a hundred thousand of some
individual's inheritance tree was extraterrestrial, then they likely
have some genes deriving from them.  In other words, a single ancestor
16 generations back (~500 years) would suffice.

Even with a "minimal mixing" assumption, I would expect most of the
population to have at least 0.1% extraterrestrial ancestry after the
first thousand years.  More likely a much greater amount, but 0.1% is
already something like a hundred times the required threshold.  Then
there's another few millennia for those genes to diffuse.

> ...or a genetest showing that a given individual has no
> extraterrestrial admixture.

There will be admixture, so such a test would be ill-advised -- unless
it was faked.

- Tim