CNBC's Scott Wapner examines the explosive rise and recent decline of direct-to-consumer DNA testing, an industry virtually unheard of 20 years ago that has convinced nearly 30 million paying customers to hand over their most sensitive personal information – their genetic code.  While industry leaders like Ancestry and 23andMe promise to reveal secrets about your heritage and health, there are growing concerns about privacy, the reliability of test results, and the question of who has access to your data.  As just a handful of companies build the world's largest private DNA databases and sell access to them to outside companies, some say it's time to step back and consider the critical questions this controversial technology raises.