breeding black mollies

Question
My daughter's aquarium started with one black molly and one yellow molly.  They have had approximately five sets of fry.  Not one of the fry has been black.  They are either yellow, white, or yellow with black spotting.  Is it possible to get black molly fry from a black molly / not black molly cross?  Or, do you need two black mollies to make baby black mollies?  We want to cross some of the black spotted mollies with an unrelated black male molly.  Any chance we will get one/some black molly fry?  My daughter is doing this for a sixth grade science fair, so we aren't too sophisticated; we just want to have an adequate explanation for whatever our results are.

Answer
Kathryn,
The problem here is that in order to breed you have to find a true breeding pair.
If the mollie that you started with was in with any other males, there is no way to tell who she bred with. They can have several sets of fry from one mating so it is difficult to say what you are going to get.
The only way to be sure is to start with babies, separate the males from females and then breed the female back to a known male.
That should get you on track to a true breeding pair.
You should be able to get a percentage of all black fry from the mollies that you are describing but without thee specific data as to the lineage there is no way to tell what that percentage would be.
If you have additional questions, please ask.