I took a graduate course at UC Berkeley a long time ago in Population Genetics,and we covered the topic of genetic markers.
Every population has certain genetic traits which are very common in their particular group or race.
It can be things you can see, or blood factors, or almost anything. So, for instance dark hair and eyes, are comon to certain people, including Indians, whereas Swedes will have fairer skin, more blond hair and blue eyes. The mongolian spot at the base of the spine someone mentioned is a very specific genetic marker for northern Asians and American Indians. It shows up at birth and looks like a bluish bruise at the base of the spine. It can vary in size from quite small to pretty large.