Has this happened anywhere in the US? If you work for an employer with group insurance, how can an insurer reject you or even alter your rates? If you don't work for a company with a group policy, buying off the ACA marketplace also is going to prevent the insurer from implementing these types of restrictions on coverage. Maybe this is something that would go on in the life insurance market but so much of the health insurance marketplace is heavily regulated in terms of writing policies and setting rates, that it doesn't appear that getting your DNA would do much for them.
Notwithstanding that slight predispositions for minor conditions could be priced into an underwriting model without all that much pain — I'm not even sure this sort of exclusion you propose has been legal in the past decade or so.
Not to mention, "sorry, <XYZ> Health Insurance won't cover you because you show a slight genetic predisposition toward fingernail cancer. Good luck."