Why does my dog have a small patch of hair missing under the eye?

Not literally on the eye but under the eye area. My friend's dog had mange before and I think my dog might have it. I showed my mom but she keeps saying "It's small, it'll go away" But I have a feeling it would get bigger. I kept asking my mom if we can have a vet look at it but she keeps saying that if it gets worse then we'll take him to the vet.

Mange is actually a mite that causes hair loss. It can be spread from dog to dog and dog to people (this is only a certain kind not all kinds). You can get a special dip from your local pet store. Just follow the instructions and the fur should grow back. If you do not treat the mange it will get worse and spread to the rest of the dogs body and to other animals. Mange is also itchy so your dog will sratch at it and it cam cause sores which could get infected.