If you have ruled out allergies, mites, mange, etc. then it is probably alopecia, which is just a fancy name for baldness. Brody has no hair on his neck, very sparse on his chest, none on his belly. His skin/coat are healthy, he just has places of missing or sparse hair. It's actually pretty common.
In blue chihuahua's and some of the other dilute colors, you may see alopecia that is severe, where the chi actually looks hairless (like a chinese crested). This is a result of the blue gene being linked to this pattern baldness.
Some people have seen a re-growth of thinning hair by using a supplement such as Nupro or Missing Link. Both are good add-ins to food, even if they don't always regrow hair. Make sure your dog is on a good quality grainless food and it's a good idea to supplement with fish oil a couple times a week for skin/coat health.
