Well, it is already known that the ability of a human to bend one of the four main elements to will is as a result of the spiritual energy they possess called ‘chi’.
What determines the element they bend?
At conception, their chi contributes to shaping their genetic information to build their body in such a way that it bends elements. Each person can theoretically bend more than one element, but so far, everyone bender aside the avatar can one bend one element. This is because, manipulation of genetic make up by chi to produce bending capabilities in a body is not responded to by the body perfectly. The human body is limited, and as such, even though the genetic makeup accounts for all bending, the body will only develop to Favor element as developing the body for all elements will put too much strain on the body.
What how does a benders body differs from non-benders?
I will do my best to explain these, as I a not a genetic engineer, I have less details of the jargon.
Firebenders. Every human have the ability to generate heat from metabolism and also to tolerate some amount of external heat. Fire benders genetic makeup have been built to generate and resist heat of very high temperature, and as such, they can easily orchestrate combustion from parts of their body they have full control over e.g legs, arms, mouth. The same gene responsible for these heat generation and tolerance is also responsible for secretion of nitroglycerin sweat, which can burst into flames with these heat.
Water benders. Unlike the average human, the bending gene in water benders made their body as flexible as contortionists, many of them even go into gymnastics as a profession. 104
Their body, not only is it flexible to mimic the motion of water, it is made up of 83% water, for comparison, the average human body is made up of 73% water. A study revealed the increased water in their body increased the hydrogen bond between their body water and the external water they bend.
Earth benders. The genetic make up of earth benders makes their muscles thinner, larger in numbers and stronger in total. This strong muscles also increase the force of attraction between them and the earth, give them the ability to move the earth.
Airbenders. The genes of airbenders codes for the development of an enhanced respiratory system. The develop larger lungs to accommodate more air, and they also possess larger vessels for air transport.
The genetic makeup of benders determine the element they bend. For some people, their gene do not code for any element. Only the avatar possess a body that possess a body with the dominant gene for all elements. 731