Drosophila grey / yellow
Since Drosophila have the same system of sex chromosomes as human beings, (XY males and XX females).  Its clear that the sex determination is important - no females have yellow bodies. This is the key observation.  

Amongst the males you have about 50%/50% Y/y phenotype.  This appears to be a classic sex chromosome related trait.  Does this help?  I can go further, but this should get you there.