Earlier quoted context omitted.
Right, no quandary at all created by the possibility that mistakes and misunderstandings might lead to children with severe medical problems. Short, miserable lifetimes of suffering are a high cost for "improving the species", and they are a real possibility if the approach taken is to just try things that seem worth trying.
Similar risks are taken when two native people from different parts of the world have a baby together.
That's not true. Most genetic disorders are caused by receiving one or more defective copies of a gene. In the case of a single defective gene causing the disorder, it doesn't make a difference what gene the other parent has. In the case where multiple defective copies cause the disorder, the further apart two lineages are, the less likely it becomes that they will each have defective copies of the same gene.
There's probably a higher chance of blood type incompatibilities, but that isn't a lifetime genetic disorder for the child, it just needs to be accounted for during the pregnancy.