Kid Flash shrugged. "DNA? I dunno." Bart's story was about as complicated as it got, but he decided to give Tommy the condensed version. "Like everyone in my family has it. My dad, my dad's sister, both my grandpas." Bart gave a one shouldered shrug at this point-- there was one grandpa he didn't want to think about very much.
Though Bart's parents marriage had ended the feud between the Allens and the Thawnes, the peace had only been temporary-- and had never permeated this far into the past. To admit to his own heritage as a Thawne in this time would like tattooing 'I'm with the bad guys' on his forehead.
"Born this way." He added, after a moment's thought. If Tommy wanted to know more, Bart would answer his questions but he wasn't going to simply volunteer anything further-- it might get way too confusing.