One could bend and one couldn't, so it was clear that genetics wasn't the primary thing that determined it.
Something can be genetic and still not automatically turn up for every descendant. Katara and Sokka had a bending mother and a non-bending father, so it's not too suprising that only one of the kids can bend. The interview mostly confirms that it's mysterious and they can do whatever serves the plot. But if I were to apply fake science to this...
I don't think genetics are actually involved (this is a world where it is possible to kill the moon. I assume inheritance happens in approximately the same "observable" way it does in our world, but that the answer is closer to "it's magic" than "there are little material objects in your body that determine your characteristics.")
Actually, cool thought just occurred to me: I'm not sure if everyone in the world is supposed to reincarnate or just the avatar. But let's assume for a moment that everyone reincarnates somehow. So you may inherit traits from three people instead of two: your mother, your father, and the person you're being reincarnated from. So you get a Punett Cubes instead of Squares. Your mother and father could contribute to your overall bending potential, but the TYPE of bending you could do might be determined by your spiritual ancestor, which explains why everyone can only bend one element, even if they had a Water Tribe mother and an Air Nomad father. People tend to be reincarnated into their own tribe, which is why Water Benders normally give birth to other Water Benders, etc.
The inheritable traits determine your potential, but your actual bending skill depends on your spiritual training.
In real life, for example, I have a naturally ability to *quickly improve* at art, but I do NOT have the ability to quickly improve at Basketball. I improve somewhat medium-ly at Tennis. So for every hour I spend practicing art, I improve 1 arbitrary unit. I improve .5 AU at Tennis, and .05 at Bastketball. They're like coefficients. Judging from the show, Aang improved very quickly when he put his mind to it (so, say, Bending coefficient of 10) and Katara only had a coefficient of 3, but Katara put in much more work than Aang did so she was better than him for a while.
I'm also betting, for narrative reasons, that Aang and Katara had one Airbender, one waterbender, and one non-bender.