This website offers tips for asthma and allergy sufferers. I found it very helpful.

It’s the number one most treatable disease on the planet. While it is possible for a person who is still in the growth and devolopment stages of their life to have their asthma get better their airways still have the underlying sensitivity and when they become older adults the asthma always comes back to varying degrees, dependant on what the specific triggers are. Asthma can always get worse especially if untreated or undertreated. A the airways become remodeled and thereby increasingly more sensitive and reactive.

I’m not sure about genetics, but asthma can get worse, and it can also get better. It’s just one of those things that can’t be predicted.