I think you could also explain this without the positive and negative qualities of the alleles. For instance, what if all benders are necessarily heterozygotes (Bb)? This would allow combinations of benders, nonbenders, and a bender and a nonbender all to produce bending offspring. Additionally, this could explain why offspring of parents who bent different elements could match either parent's element because the bending allele with its associated element would be inherited only from one parent of the bending child. What do you think?
Haha This must of taken alot of thought. I doubt the creators really cared about genetics though, they probably just were like this character should have this bending ability. And as you said, genetics are not so cut and dry as the Mendel you learn in biology 1. It can get really complicated. But I like to believe that, as bending is a fictional ability, it is at least basically free from the rules of genetics. Two nonbenders from the earth kingdom can't give birth to an airbender but they could have an earthbender, genes or not, if the creators want.
An airbender would have to marry a non-bender that is not a carrier to have off spring that are 100% carriers of the bending gene, meaning they will be unable to bend but their offspring will be benders, if two carriers marry.
If an airbender married a carrier, then you would have 50% chance to be a bender and 50% chance to be a carrier.