Genasi

Sometimes mortals and genies intermingle and genasi are their offspring. This is however not the most common way for a genasi to be formed. The planet Daovis is filled with elemental influence which produce genasi. Areas where the planes overlap or the influence is a common cause. On a new moon of Lycas genasi can be born with the element of the corresponding month in the Dentah Calendar.
Genasi are often pictured as only human descended but this is far from the truth. Humans do produce more genasi than other races but all races can become genasi.
When genasi have children whether with non-genasi or genasi they can have a genasi child. This is however far from a garantie. Two genasi parents on average have 40% of their children show elemental influence. And over time a family that had once had elemental influence will loose it again. But the genasi blood can lay dormant in the family line and randomly show up again. But most scalars agree that on average there are no more than 3 generations of genasi if no other influences ocur and the two original parents were both genasi.
For each of the 10 elements a genasi of that type can be born. Some classify obliterators as genasi but their anti magic is often seen as the direct opposite of elemental influence. More information about the specific subspecies can be found by their respective Elemental Plane.

Genasi and Primordial

Genasi always learn their own dialect first and then 3 of the others from connected elements. For example fire genasi often learn the radiant language Luxan and auran.

Places with higher rates of Genasi

Azor

Higher rates of fire genasi especially around the fire eyes pass. Earth genasi are born more around the cliffs that divide the land. Water genasi are more likely in Maiko the City of Rain but this is a slight effect in comparison with the fire and earth genasi.

Aprax

The fountain of lillies produce water elementals. And some scolars suggest that the Ember Born are a type of fire genasi. Normal fire genasi are also found in Avsi.

Playing a Genasi

When creating a Genasi choose a Genasi subrace and a parent race. You gain the benefits of the Genasi race and subrace. In addition you can choose a trait of your parent race and a skill or language that your parent race gets acces to, as long as it doesn't give acces to a spell or feat. For example if your parents are of elven decent you might choose fey ancestry, darkvision or fleet of foot and then for a skill you can choose perception. 
Some racial traits might be too strong so please discuss your choice with the DM. If you choose one of the small races you might choose an extra small ability in turn for taking the reduced speed and size that accompanies it. 
Human Genasi gain an extra plus one in a trait that they do not have a bonus in yet and an extra skill or language. So for example a human air Genasi can gain a plus 1 to dexterity, a plus two to constitution and a plus 1 in intelligence.