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There are different types of combs, I don't mean to sound condescending, I just don't know how much you know about chickens.  The Dominique should have a rosecomb like newgoosegirl posted.  Then the Rocks and Australorps, as well as many others, have a single comb like the RIR that looks like a straight ridge when they are chicks with little baby spikes along it.  The rosecomb or pea comb will be not as long and wider than the singles, if you compare the black chicks, I think you will see what I mean.
What I meant in my last post was that the little one could be a Black Rosecomb which is a breed.  If you got them from McMurray, they have all of those and you could have ended up with a Black Rosecomb as a free chick.  And like so many want to say is always the case, it could be a cockerel.  However, I have gotten chicks from MM and I don't always get cockerels, though you are more likely to get them.  Maybe your mother's friend or whoever got them mixed up when they separated yours out.
As for Barred Rocks, there is a spot on the top of the head that you can tell the males from females, but I can never remember which is which.  I think because of the other post that the white spot means cockerel.
I had a hard time the first few days trying to tell my Black Australorps from my Barred Rocks, but as soon as their feathers started I knew very well who was who.
Like I said before, wait a few days when they get more feathers and we will be able to sort out the others, I think.  The non black ones though, I am positive I told you correctly.
I hope that helps.