[Closed] The boy-girl puzzle

As an alternative to the old plane-conveyor one...

You ask a woman how many children she has and she says two.
Then for some odd reason you ask her [i]"Is at least one a girl?"[/i], to which she replies "Yes".

So what are the odds that she has a girl and a boy?

(to avoid some argument: assume this is a perfect world where the odds of boy or girl conception are 50/50 and that there are no genetic "third sex" possibilities)