One simple and effective way, when in doubt if a fragment of the argument is conclusion, is to perform WHY test. Let's see, we doubt the first sentence: "Certain genetic diseases are more prevalent among certain ethnic populations."

Now let's perform the WHY test and look for answer in the rest of argument ...

WHY Certain genetic diseases are more prevalent among certain ethnic populations?

If some one is able to aanswer this question by using the below quoted statements, then Yes we have the argument. To me, there's no argument.

towerSpider wrote:Certain genetic diseases are more prevalent among certain ethnic populations. For example, Tay Sachs disease, a usually fatal genetic condition caused by the build-up of gangliocides in nerve cells, occurs more frequently among Ashkenazi Jews than among the general population

hi. the book (manhattan) says that there is no conclusion in this stimulus but to me the first sentence very clearly and evidently appears a conclusion. what's your take? thanks.

The first sentence is a statement of fact, which is in turn supported by the example.

In an argument the direction of the reasoning is from the facts/evidence to the conclusion. You could say that someone, at some time in the past, reached the conclusion that genetic diseases are ethnically-linked by looking at examples such as Tay-Sachs, but the way this argument is presented, that conclusion is long past and we are presented with the statement as fact.