Is it normal to get gray hair in your twenties? Is it a one-time thing or a sign of more to come?

I’m 22 and just found my first strand of gray hair. (Sob) It’s actually half white and half black because it started to gray midway. I am scared; my dad started to gray in his 20s and now is almost all white in its mid-50s.

Could this just be a one-time thing, or is it a sign of many more grays in the near future?

Must…fight…genes…