Bryan Kehl leans back, smiles and says his dad has always been a "storyteller."

He wraps exaggerated finger quotes around that last word to make it clear the stories his father tells aren't always rigorously fact-checked.

Six of Gary and Nancy Kehl's nine children were adopted, including Bryan. As a rule, there were never any distinctions made about whose genes came from where: Everyone was to be treated equally. But there was always one exception—and it always involved Bryan.

Giants linebacker Bryan Kehl, above after a preseason win against the Patriots, was adopted. He didn't find out until last year that his father was NFL journeyman running back Maurice Turner.

In one of those yarns that his dad was so fond of telling, Bryan's biological father was an NFL running back. "I wasn't sure it was true," he says. "Like I said, my dad tells stories.''

The first time Bryan touched a football, however, out in the family's backyard in Salt Lake City, he decided to play running back. All through youth football and until he went to high school, he was a running back. Now, some 20 years after that first run, he thinks his fondness for carrying the ball was influenced by family lore. "I'm sure that had something to do with it. My brothers all played defense."

Mr. Kehl is a strapping 26-year-old linebacker entering his third season for the New York Giants. He's no longer a running back. He plays defense like the rest of the Kehls.

But when it comes to his path to the NFL, there's something basic that Bryan Kehl doesn't know—whether his athletic talent was something bred into him by his parents, his siblings and his Utah childhood, or whether it was coded into his DNA.