Heartburn doesn't have any relationship to the race of the baby, it is an old wise tale that heartburn means your going to have a child with a lot of hair, but that's just an old tale. Heartburn comes from the lack of space between your stomach and chest due to baby, the acid from your stomach gets pushed back up into the throat cause the acid reflux, and usually the bigger the baby gets the more it seems to happen. The only way you can be 100% sure is to have a DNA test. Don't try to base it off of skin color or physical features because that's not 100% accurate. Good luck!
The only way to determine the father would be a DNA test. You can't tell if your baby is mixed by getting heartburn or any other symptom..