A single sperm donor can have hundreds of children. Some of those children end up wanting to meet their biological father, and any half-siblings. Their search is the subject of a documentary that airs tonight on Rocky Mountain PBS. It’s called “Donor Unknown: Adventures in the Sperm Trade." It follows 20-year-old JoEllen Marsh, who finds her donor father on an online registry. The registry was started in 2000 by Wendy Kramer of Nederland. She also makes an appearance in the film. Kramer conceived her own son using donated sperm. She spoke with CPR's Andrea Dukakis.  [Photo: ITVS - Independent Lens]