It's common knowledge in the Avatar universe that only the Avatar is capable of wielding and mastering the four elements. Meanwhile, all other benders only have control over a single element, while some cannot bend at all -- which propels the title of Avatar to a legendary status.
Though fans have long since discussed whether normal benders can learn to bend more than one element, according to canon material revealed in The Legend of Korra, the answer is clearly no. However, Avatar: The Last Airbender revealed that they are still capable of much in regards to sub-bending. In fact, some characters can use multiple forms of sub-bending, like Toph who can metalbend, sandbend and use seismic sense -- all of which derive from earthbending. So, why can't normal benders bend more than one element, and how are these abilities passed on?
Elements vs. Sub-Bending
In Avatar, bending is a special ability allowing an individual to manipulate one of the four physical elements: water, earth, fire or air. Each bending style is used as an extension of oneself, something achieved by mastering the martial art style associated with the particular element. Of course, mastering an element is easier said than done and only those skilled enough can unlock a specialized bending technique.
These techniques are sub-skills pertaining to a primary element. Each requires a certain level of mastery before the bender is able to branch off into one or more sub-bending techniques, while some also require a deeper understanding of the element or even other types of bending. These subcategories include waterbending (bloodbending, healing and spiritbending), earthbending (metalbending, sandbending, lavabending and seismic sense), firebending (lightning, lightning redirection and combustionbending) and airbending (flight and spirit projection).
Some forms of sub-bending are easier to acquire and are therefore more common -- like sandbending and metalbending -- whereas other skills like bloodbending and combustionbending are more unique and much harder to obtain. Although it is possible for a bender to obtain multiple sub-skills, like Toph in The Last Airbender, most can only obtain one. It depends on their skillset. For example, Bolin trained desperately to unlock metalbending but discovered that he's better suited to lavabending.
What Happens When Benders of Two Different Elements Have Children?
According to the lore of Avatar Wan, after fusing with Raava -- the spirit of peace and light -- and becoming the first Avatar, lion turtles ceased to grant humans with bending abilities. From here, it's implied that bending was passed on through genetics. However, the ability acts more like a recessive gene since two non-benders can give birth to a bender -- like Katara and Toph, who both had non-bender parents.
When two benders of different elements have children, those kids can inherit either element or none at all. In the case of Mako and Bolin, who had parents from two different nations, Mako inherited firebending from their mother while Bolin inherited earthbending from their father. While it's unknown if their parents were benders, they each carried recessive genes from the Fire Nation and Earth Kingdom respectively. Additionally, while Kya and Tenzin inherited an element from each of their parents, Bumi did not and was a non-bender for most of his life until the Harmonic Convergence.
The power and talent in the element that's passed on varies, and it's not guaranteed that one powerful bender will give birth to another. That being said, sub-bending abilities are not passed on genetically since they are determined by an individual's own skill and talent. Rather, hard work and practice is the only way to obtain a sub-bending ability.
Why Only The Avatar Can Bend Multiple Elements
As mentioned, normal benders in Avatar cannot acquire more than one of the four primary elements. In the Legend of Korra episode "Beginnings, Part 2" a lion turtle explains that the human body is incapable of holding two or more elements at the same time. To combat this obstacle in an attempt to defeat Vaatu -- the spirit of darkness and chaos -- during the Harmonic Convergence, Raava holds onto the excess elements, while Wan trains to master them one at a time. As such, Wan can only use one element until Raava passes through him, swapping one element out for another.
Switching between elements proves to take too much time in the heat of battle and doesn't offer the power necessary to face Vaatu head-on. Wan convinces Raava to fuse their energies together, and eventually takes advantage of the Harmonic Convergence to merge their spirits permanently. This enables Wan to wield all four elements at once as he successfully defeats Vaatu and restores balance to the world. This created the Avatar state and thus, the first Avatar is born with the fundamental aspect needed to master all four elements being Raava.
