Some people define ethnic identity primarily in terms of DNA. In other words, they look at body chemistry, body configuration and physiology as the main determinant of ethnic identity. This is a problem because your genes don’t carry your culture.
Culture is something that you learn. Culture is a function of nurture not nature. Just because somebody physically looks like they come from a specific community doesn’t necessarily mean that they have all the cultural attributes of that particular community. It also doesn’t mean that they naturally care about such cultural prerogatives and concerns.
Other people define ethnicity in terms of culture, so instead of focusing on what you look like, they focus on a community of concerns and sensibility. This is crucial because you can tell somebody comes from a specific culture based on how they look at the world and how they see themselves in that world.
When you are trained as a little kid to look at the world in a certain way and define certain big concepts in life, like what is the good life, what is good, what are the things to strive for in life, and so on and so forth, you walk away with the distinct take on the world and an attitude about it that nobody can readily take away from you, unless they brainwash you. This is the definition of an ethnic identity we subscribe to, and accordingly, we work hard to keep that alive.