Fruit flies have four major chromosomes.  Fruit flies also have a number of body markers that are useful for mapping genes to given chromosomes.  Curly O (Cyo), located on chromosome II, is a dominant marker mutation which causes the wings to curl on the ends instead of beings straight.  Chromosome I is the X chromosome, which has eye color alleles Red (dominant) and white (recessive).

You find a new dominant mutation that causes a purple abdomen and decide to map it to a given chromosome.  You perform 3 crosses:

Cross 1: Purple males with red eyes to females with white eyes.  You get the following phenotypes in the offspring:        

Cross 2: true-breeding purple to true-breeding Cyo.  All offspring have purple abdomens and curly wings, so you testcross them to wild type flies.  Here are the F2 offspring:                

Cross 3: true-breeding purple to truebreeding Sb. All offspring have purple abdomens and stubbly bristles, so you testcross them to wild type flies.  Here are the F2 offspring:                

Which chromosome is your purple gene on, and how can you tell?  And how do you know it is not on Chromosome IV?

I've worked out every crosses and reached the 3rd one. the only thing that doesn't seem to make sense to me is the ratio of cross 3.

I was thinking of parental-recombinant but the ratio doesn't make sense if it's parental-recombinant just purple and just stubbly should be 20 each since they are the ones that are different from their F1 parents.

What situation might account for that kind of ratio?