Can you trap and count enough flies to work out the parents' genes? How many do you need to sample before you're confident in your answer?
Start with the animated presentation to introduce the concept of red- and white-eyed flies. It covers dominant and recessive genes, alleles and punnet squares.
Then use other two Flash files (Pop 1 and Pop 2) with animated populations of flies. By sampling and calculating the proportions of each type of fly, pupils can suggest the genotypes of each parent.