It's a bit trickier than that. Genetic problems, per se, are only half of the issue. The other half is genetic diversity.ShirouZhiwu wrote:I've always found ST approach to genetics terribly flawed. With the technology at their disposal, identifying and fixing genetic problems should be easy, even if they only had 2 people to start with.
If you have too small a population, it becomes increasingly likely that some particular genes will be lost, simply by random chance. If (for example) there's only one person carrying a particular allele, and he (or she) has two children, then there's one chance in four that neither of the children will inherit the allele, and it becomes extinct.
This sort of low-population genetic "bottleneck" reduces the genetic diversity of the population, and it doesn't recover (except very slowly, as a result of mutations) after the population increases again. A population can end up being genetically quite uniform - not as much as clones, but not far from it. Even if all of the genes are "healthy" ones (no lethal or harmful alleles at all) the species can end up very vulnerable to disease and environmental change, because they''ll all have the same types of resistance or lack-of-resistance. A lot of our carefully-bred mass-farming plant varieties are vulnerable to this - all it takes is one new mold or bacterium coming along, and it can wipe out an entire crop all across the world because none of the varieties being farmed have any resistance to it.
(There's evidence that the human race has been through that sort of bottleneck. A tribe of chimps or bonobos, all living within 20 miles of one another, have more genetic variation within that small group than the entire human race has in many of its genes.)
To get around this with genetic engineering, you'd have to create a complete "gene bank" of the species, and then either manually "program" the genes of every new generation, or re-introduce endangered or extinct alleles every few generations to counteract random losses. Either approach would be expensive, and prone to totalitarian abuse.