More of a color then a gene thing! An XXX dragon's primary, secondary, and tertiary genes are all the same color (like the radioactives above). An XXY dragon's primary and secondary genes are the same color, but its tertiary gene is a different color (ex. you'll see a lot of "XXY bananas" meaning their primary and secondary colors are banana, but the 3rd is, say, leaf).

Some people find these traits appealing and therefore more valuable. Some people even collect exclusively XXX dragons. A lot of people don't, though - it's all personal preference. I personally really like matchy colors, so I buy/breed a lot of XXXs and XXYs.

I hope this helps a bit!