GB,
Please note that the person who called
descendant of an idiom also misspelled
descendant!
In comparing the combinations
descendant of and
descendant from, it would be correct to say that
descendant of is the
more idiomatic usage. This does not mean that
descendant of is an
idiom. Nevertheless, sometimes people say
idiom when they mean
idiomatic usage. It's just a matter of being in a hurry when responding to a post, I suppose.
An
idiom is a group of words which has a meaning that cannot be
known simply by knowing the literal meaning of each of the words
contained within it. For example,
kick the bucket is an idiom which means
die. Even if we know what
kick means, and we know what
bucket means, this is not enough to know that
kick the bucket means
die.
I think you can see that this is completely different from the problem
of choosing the proper preposition for a given noun or verb, which is a matter
of
idiomatic usage.
CJ