Gastric juice does not typically cause a burning sensation within the stomach. but reflux of this gastric juice into the esophagus can cause a burning sensation. one reason why this occurs is that the esophagus

The answer would be: secretes mucus that is not identical to stomach mucus

Stomach cell lining is protected by a thick mucus that prevents the acid from destroying the epithelial cells. But the mucus that secreted by the esophagus is different than the stomach. That makes esophagus can't protect itself from the stomach acid. 

4, two dominant and two recessive