There is no perfect genetic tell.
In poker, a tell is something you do that gives away the fact that you're bluffing. Maybe you itch your nose every time you have a bad hand. So now when you make a big bet and itch your nose, you'll lose to the players who have caught on. Your nose itching is your tell.
In an article about the 4 percent of men who are raising someone else's kid, Men's Health lists what it calls genetic tells. Instead of nose itching, these tells are traits to look for in a child who might not be yours. The idea is that in a simple genetic world, some traits are impossible for certain parents to pass on. So dad is supposed to use these traits as a tell to uncover mom's bluff about paternity.
The problem, though, is that none of these traits are a sure way to exclude paternity. In fact, most of the traits they list can be safely ignored. Light haired parents can and do have darker haired children. Parents without a cleft chin can have a child with one. Parents with attached earlobes have kids with unattached ones. And tongue rolling seems to hardly be genetic at all.
All of these exceptions are actually pretty common. The one trait they discuss that has some validity is blood type and even this one isn't bulletproof. For example, in Japan, an AB man had an AB child with an O wife, which conventional genetics says is impossible. But the dad really was the dad. To understand how this happened, we'll need to quickly go over blood type.
