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Wow, i will try to describe this as best I can. 1st just to be sure u know, incomplete domoinance in Biology means that a medium of 2 traits will be displayed, example, a red+white flower= pink offspring.

That being said, this is how you set up the punnet square:

Every gamete MUST have an R,B and S, the size (big or small) is arbitrary. If they are not linked and can assort independently, make all the combinations you can of RBS for each parent.

To make things easier I'll just give you the combinations:

You said you have: RrBbSs x RrBbSs

This means both parents can make the following gametes:

RBS, RBs, Rbs, rBS, rBs, rbS, RbS, rbs

Make these combos go horizontal and vertical and do the punnet square as normal.

NOW, because this is incomplete Dominance, this means if for example rrbbss= white and RRBBSS= red, when you do the punnet square, the more CAPS (RBS) you have, the more red the offspring will be, the more lower case you have (rbs) the more white the offspring will be.

HOPE THAT HELPED! GOOD LUCK!!! (^_^)