Without knowing it, we're all in the gene databases already
I have written before how future technology affects our privacy decisions today. DNA collection is definitely one of these areas. As you may know, law enforcement in the USA is now collecting DNA from people convicted of crimes, and even those arrested in a number of jurisdictions -- with no ability to expunge the data if not found guilty. You may feel this doesn't affect you, as you have not been arrested.
As DNA technology grows, bioinformatics software is becoming able to determine that a sample of DNA is a "near match" for somebody in a database. For example, they might determine that a person in the database is not the source of the DNA being studied, but is a relative of that person.
In a recent case, a DNA search turned up not the perpetrator, but his brother. They investigated the male relatives of the brother and found and convicted the man in question. As technology improves, it will find not only sisters but 4th cousins. People have, I think about 8-10 first cousins and siblings on average, but the number of 4th cousins approaches 1000 -- 16 sets of great-great-great-grandparents (p^5) each with about 50 g-g-g-grandchildren (c^5). It stands to reason that a database with 1% of the population will have several of your 4th cousins and probably a 3rd cousin or 2 in it. (Numbers based on 2.2 kids/couple which is a modern number, the real number's higher.)
The problem is that the various 4th cousins they find will be via different p^5s. Your family may be the only one with that specific set of ancestors. So by finding your distant cousins they find your family. The better they get at determining relationship distances over time, the more accurately they can do this. (Many of today's techniques require strict paternal lines for Y chromosomes and strict maternal lines for mitochondria, but they're getting better all the time.)
Add to this the private databases of DNA that will be built as people investigate their own DNA or that of their newborns, and you get closer and closer to a sure thing.
So while you may not consent to be in these databases, you will be in them anyway. And a decision to scan your own DNA may well affect your family members just as much as yourself.
Because DNA is one of the involuntary biometrics -- you leave it everywhere you go, it is one of the most troubling. I'm not a big supporter of trying to protect privacy with laws but I often feel DNA may have to be an exception. We probably will need laws at the very least protecting private DNA records, done for medical purposes or research, from use by the police, even if it would solve crimes and catch terrorists and pedophiles. The alternative may be Gattaca.