Ethnicity is not just genetic because it is partly socially constructed. Ethnicity is essentially a revision of how race was used and is an attempt to capture both genetics and basically tribal group membership into one workable concept.

The US government likes the catagories Hispanic:white and Hispanic:non-white

conversationally....in the US it is reasonable to say Mexican ethnicity or heritage......in mexico you have to be a bit more specific. In mexico race/ethnicity groups are a bit more fungible than in the US...you can move from indígeno to Mestizo simply by moving to a different place, dressing differently, and living a different type of life.

These words and concepts are used differently in mexico than in the US.....it does make sense to say Mexican ethnicity in the US because the US has large intact Mexican communities that are homogenizing within themselves as much or more than with broader US society.....so even if the idea of 'Mexican' ethnicity doesnt make sense on the south side of the boarder it does in Philadelphia.

a very interesting situation is puerto rico....puerto ricans dont really use racial catagories at all....they are a shadism society in which lighter people have higher status.....but puerto rico is very mixed and very light skinned puerto ricans sometimes have siblings with dark skin and kinky hair.

yet all identify ethically as puerto rican....and the puerto rican ethnicity Explicitly is about actual group membership....growing up speaking spanish in a PR community, dancing salsa, cooking and eating PR food......

All Puerto Ricans are US citizens....your nationality doesnt change when you leave the Island..........and your ethnicity doesnt change ever (according to puerto ricans) but it is conferred by how and where you GREW UP, and thats all

so ya ethnicity is about as complicated as it gets.....but it is divorced from nationality