advertisers already do target based on race and sex, genetic data will simply increase the accuracy of making those classifications

While it's also possible that advertisers could use more sensitive information, such as a predisposition to gambling/obesity/alcoholism, the signal from DNA that an advertisement is likely to be effective is still probably weaker than a person's actual habits. A very unethical company could use DNA to know how to target to make "the first hit free", but I'd like to think that such egregious behavior would result in significant public backlash and possibly the introduction of regulations