Subject: Why are certain diseases inherited, hence genetic ?

Hello, (please excuse my poor english)

Wikipedia states that natural selection "is the process by which certain
heritable traits—those that make it more likely for an organism to survive and
successfully reproduce —become more common in a population over successive
generations". Moreover, I have gathered from this very site that natural
selection is believed by the overwhelming majority of biologists to be the main
drive for evolution among living organisms. Therefore, I am asking myself:
1) Why do genetic diseases exist (or, should I say, seem not to disappear)?
2) Does it mean human beings, given a relatively similar environment on a long
period of time, will attain "perfection" (for that given environment) ?

That being said, I'm also wondering why biologists never talk about "gene pool
ratios", that is (so I put it) the proportion of a specific gene pool compared
to another. It would be a useful concept in the determination of the genetic
evolution of a people, supposing it had to go through frequent processes of
interbreeding with genetically-different peoples. I mention that concept,
because I believe it might be as important as natural selection itself. I'll use
an example: say, 10 type A men live in a very hot climate and have evolved so as
to be efficient and adapated to that environment; then say, 10,000 type B men,
adapted to colder climates, settled after a long travel among type A men, and
mixed. Doesn't it seem obvious that on the middle-term type B traits will
prevail to such an extent that type A traits will be wiped out ? And that
certain adaptative wonders developed by type A men might never reappear ? Hence
indicating that "gene pool ratios" have as much importance, if not more
importance, as the concept of "natural selection" ?

Thank you for answering me,
Frederic.

Re: Why are certain diseases inherited, hence genetic ?