Just as a tiefling is the result of devil blood in a human, a genasi is the result of an elemental's bloodline mixing with that of a human. This leads to odd traits developing. Exactly how this works depends on the elemental bloodline.
AirThese trace their lineage to an air elemental such as a djinni. They are often wild spirits driven to wander and explore. Their priorities are often fleeting, and they are prone to change at any given time. While the nature of air genasi might seem to be one of chaos, they are mostly focused on being self-reliant and able to weather out even the strongest of storms. They are just often unable to find rest anywhere for long. Air always seem to be moving around air genasi, giving them a windswept look. There is always air in their lungs and they can even make the air around them push them upwards to help them levitate for a short while. Their skin is usually pale and weathered.
EarthThe hard-working and dedicated earth genasi descend from earth elementals such as Dao. They are often more structured and determined than their fellow genasi, as well as more physical in their outlook. The skin of these genasi might look cracked like dry dirt, be constantly covered in sand, or have a rock-like texture to them. The tone is one of sand or rock and their eyes are either grey or dark. Their affinity for stone makes them extremely sure-footed and able to traverse any rocky terrain with ease. In fact, they can make the stones themselves help hide them if need be.
FireThe most passionate of the elements often have trouble keeping their emotions in check. This might be positive or negative depending on the individual. While their passions differ greatly, all of them will have an urge to dedicate themselves fully. Fire genasi's skin is a fiery red or orange and their hair might even seem aflame. Or their hair is black as charcoal. Heat is natural to them and they are even able to produce fire around them to a limited extent.
WaterFor water genasi, the elemental in their bloodline was a Marid or a similar creature. The same mercantile instinct tends to show through in these beings as in those they herald from. Or at least a longing for the sea. The skin of water genasi is often tinted towards blue or green compared to their human ancestry. Their hair can be seen waving slightly as if submerged in water and on some, water might slowly form on their skin. All of them have the ability to control water around them to some extent and many can create some water, seemingly out of nothing.
All genasi are very much shaped by the element they are tied to, be it fiery red skin, or always wet hair. Precisely how this is expressed varies a lot between the different forms of genasi and between individuals.