I know nobody cares, but when Marrow first appeared as her teenager/young adult self, Dark Beast noticed that she was one of his "creations".
So it has always been open if she is a natural born mutant he experimented on or if she was a tube born artifical creation he made. Her mother in X-men Origins: Gambit certainly didn't resemble her and could have just adopted her. Also it would make sense that Dark Beast would give useless but functional creations to the Morlocks to raise them, since it would allow him to come back and experiment on them later.