Ethnicity is less genetic or biological than geneological. Genetic differences between ethnic groups are generally either minimal or non existent, and ethnicity itself is fluid enough that claiming it as a biological thing is problematic. What ethnicity would you consider someone with German, Chinese, Irish, and Botswanan ancestry to be, for instance? And how do you know their German/Chinese/Botswanan/Irish ancestries are themselves homogenous, enough to ascribe a genetic value to them?

Basically I think claiming ethnicity as a genetic or biological thing is problematic due to precision issues among other things (like the tendency of 'scientific racists' to use it as cover for their beliefs).