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Not with an absolutely perfect record over more than three thousand years of being embedded within a highly mobile non-pureblood society, it isn't.
On the contrary. A perfect record would make the existence of pureblooded Terrans more likely, since it permits interested groups to self-segregate from individuals they deem "contaminated" even if there are no visual cues to distinguish them, because of the low content of extrasolar genes.
>It only takes a single child more than about fifteen generations back to be non-pureblood -- knowingly or not, through any of hundreds of plausible scenarios -- for an entire inbred community to end up having been descended from that child and no longer pureblood Terran.
And there are a myriad of plausible scenarios for some of the literally billions of Solomani, some of who are highly xenophobic and endogamous, to avoid getting mixed. Such as the aforementioned ability to to reliably tell what the ancestry of a given individual is. According to some research I read (such as dr. Gregory Clark's books on social mobility and ethnoeconomics), there's very little mixing between the castes of India, on the scale of millennia, despite them literally living next to each other. I would imagine that with advanced genetic technology and the gulf between the stars, most of the population of Terra itself would be pureblooded in the full sense of the word.
>The time period in question is more than a hundred generations, through dozens of major social changes and multiple interstellar wars. Such "pollution" would probably have happened at least thousands of times over, and any one of those incidents would be sufficient.
Such "pollution" would be largely restricted to social groups who do not have a cultural taboo against mating with members of outgroups - and who the ingroup is can be very, very narrow. Consider that there are numerous examples of highly insular societies today who have maintained their genepool distinct (to the point where they are the sole sufferers of a number of genetic ailments, due to extremely prolonged periods of inbreeding) despite being in constant contact with the rest of society - such as the various types of Gypsies, the Jews and the Amish.
>That's only the beginning, really. There's also things like diseases that can carry DNA fragments from one host to another, and the whole thing just gets ridiculous.
The ridiculous thing is to claim that in all the hundreds of billions of Terrans who lived through the ages, no lineage has managed to maintain purity, with the advent of tools that allow them to verify this. The Indians did an excellent job in direct proximity and with only TL 1-2.
>I mean sure, as a fictional premise or a political claim it's usable,but like many such absolutes it falls apart if you look at it closely enough. It's one thing to have records that state that all your ancestors have always been purely Terran for four thousand years. Having all those records be totally complete, correct, and not to have any other form of extraterrestrial contamination in your genes is quite another.
In order to effect the kind of universal genetic spread, the non-Terrans would have to flood Terra by the billions themselves and then somehow convince every last single ethnic/religious/social group that miscegenation is the way to go. Good luck with that.
>By the time of the Third Imperium, there are no longer any truly pureblood Terrans. There are just those who might like to claim such a thing.
By the time of the Third Imperium, there are plenty of truly pureblood Terrans. It is an extraordinary claim - requiring extraordinary evidence - that there wouldn't be any.