punnett square help

After performing a series of breeding experiments, the naturalist sets up a mating between some pure breeding red and a pure breeding yellow lizardos.  Of the 100 F1 offspring that hatch,  73 lizardos are red, while 27 lizardos are yellow.  Choose a letter to represent this gene.  Fill in the Punnett square to illustrate the expected outcomes (not numbers, just the letters) if a yellow lizardo were mated with a red lizardo from the F1 generation.