Well, ethnic groups certainly exist. There is some use in considering ethnic groups, for example, some have higher risks for certain illnesses than others, etc. Nevertheless, these ethnic groups are not separate entities, especially nowadays, with our global civilization. While in the past, there were many human populations that formed truly isolated breeding groups (such as Japan, certain tribes in Africa, etc.) this is now rare. One can look for certain genetic markers, but it can becomes arbitrary which are to be associated with which ethnic group, and which should not. An individual can have markers for muliple ethnic groups.. what do you do with them? Also, I would emphasize that if such groups interbreed with others, and individuals within a group vary more genetically than the groups themselves do, then the importance of such groups becomes questionable.