yeah but imagine zuko and katara having three children and like one is a firebender and one is a waterbender and their third kid is a nonbender but, like, zuko makes absolutely sure that their third kid gets absolutely the same love and respect as their other kids and teaches them how to be an absolute ninja BADASS who can actually go toe to toe with their bending siblings in a fight and they all spar together as a family and the kids come up with combined attacks on the sly to impress their parents and it’s just AAaAAAAAAaaaaaaa
Bo can lava bend because he has fire in his blood, he came from a mixed family. Earth Kingdom dad and Fire Nation mom, even though he’s an earth bender he has his mom’s fire bending gene. It’s said that only the avatar can lava bend because it’s a mixture of earth and fire. Up until Republic City, mixed families were almost nonexistent- so until then there probably wasn’t any normal person who could lava bend. Ghazan must also have an earth bender and a fire bender parent.
There are three combinations from a mixed family, element A, element B, or a non bender. Each combination has a recessive gene, like how Bolin has the recessive gene for fire bending.
Say Kya or Bumi had a child-
They each have a recessive gene that they’re able to pass on to their child, either for air or water (the element that they don’t bend).
Like if Bolin and Opal have kids, one of them could be a fire bender.