i think kataras parents are non benders

If it was a recessive gene, then how could two benders like Aang and Katara have a nonbending child? Let's say B = nonbending and b = bending. Aang and Katara would both have to be bb type in order to be benders. Therefore, all their children could only receive bending genes. All of their children would be bb.

There'd need to be more than one bending gene for each element. Say A and a, F and f, W and w, E and e probably along with the B and b genes. They could potentially have the right genes to be a 'bender' but the elemental genes don't enable them to be.

so their child could be AaffWweebb which means they are a bender, but because it's not AA or WW, they can't bend.

I've never studied biology before, I just saw a video about this on youtube once.