The only time I've given my DNA was when I was tested to be a bone marrow donor for my sister; they also did further analysis to test for familial genetic problems which did show up as positive. I would only do this for very specific reasons that I initiated or were initiated by my own physician, and only at very well-known, reputable institutions. I would also expect a contract outlining exactly what my genetic information was being used for and what conditions were placed on that information. When I was tested, everything was carefully outlined for me and explained.

I'm wondering where they got your name and/or why you were asked. Sounds very random which would make me very wary.