If you watch “C.S.I.,” you know about DNA. Just a few cells from a blood stain, a light switch, even a toothpick, is enough to identify the bad guy, or set the innocent free. At least, that’s how it works on TV. In real life, it turns out that what we think of as infallible evidence may not be so reliable. Madeleine speaks to the author of a new book about the dark side of DNA.