First thing first. The gentics are based of Rh factor and Multiple Alleles ( which I dont even know can coexist) but I havent done much of biological research just show research and shower thinking. Ok thanks.
Edit: I feel need to mention (to clear some confusion), Not everything in the Avatar world can be explained with biology that is why I had to put the spirts into some of this, for without the spirits bending would not exist in the first place.
Edit: This theory is excluding factors that could change how powerful a bender they are; such as, enviromental factors, and mutations (except for the avatar). Also it's the assumption that once the child is born the parents are aware that the child is a bender
MAJOR CHANGES COMING: Reason being, I wont be shower thinking this through now, I will bring out everything I know :)
- = bender
- is co-dominant to +
W,F,E,A are all co-dominant
When I mention powerful, I mean from birth not after practice
W-W+= Waterbender or nonbender
W-W-= Powerful Waterbender (I.E Korra)
F+F-= Firebender or Nonbender
F-F-= Powerful Firebender (I.E Azula)
E+E+= non bender
E-E+= Earthbender or Nonbender
E-E-= powerful Earthbender (I.E Toph)
A-A+= Airbender or nonbender
A-A-= powerful Airbender (i.e Aang)
What if two diffrent types benders get together?
There are two possiblities. If it is - - then the child dies due to the child not being able to handle so much power. Only the avatar (which will be explained why) can
the other possibility is if it is + - then, due to spirits having a noticible and logical impact on the world, the spirits choose what type of bender it is. If there are more kids then if one is, in example of Mako and Bolin, firebender then the other WILL be earthbender or nonbender. (if the parents were of F+F-, E+E- genotypes or similar to make an outcome of + -)
Example of two benders not having a bending child is Bumi, Aang and Kataras son, who most likely has a + - genotype and it was chosen he wasnt a bender because kya, his older sister, was a bender and his younger brother is
I dont get the + - thing
Like said before, it depends on the spirit but if there are multiple children with the same genotype then it "switches of" between the choices
What about the avatar?
The avatar is a - -, and due to the avatar cycle the next element in the cycle, but I think the Avatar, from a biological standpoint, the avatar also has a chomosomal mutation of some sorts allowing him to contain more power so he or she can bend other elements.
If you have more questions please ask and I will answer them the best I can.