Quotes from R.L. Trask

Sex is a matter of biology, while gender is a matter of grammar, and there is no earthly reason why sex should be involved in gender distinctions.

Note in particular that a noun denoting a group of people takes which, not who. You cannot write *the battalion who had captured the fortress because a battalion, though composed of people, is not itself a person: write the battalion which had captured the fortress.

It is possible to use that with people, but the result is often rather clumsy. While the linguists that are working on this problem is not quite wrong, it doesn't sound as good as the linguists who are working on this problem. Prefer who with people.

dissociate, disassociate Both are possible, but dissociate is more usual, and is recommended.

it is impossible to use that if the relative clause is non-restrictive – that is, if it does not serve to identify the thing under discussion, but only serves to provide more information about that thing. So, you must write the Suez Canal, which was opened in 1869, and you cannot write *the Suez Canal, that was opened in 1869.