In theory, you could rule that the distant descendants of pairings between humans, and half-elementals, could also be genasi. In the same fashion as half-fiends eventually producing tieflings.
Still, normally, they are genie descendants, not elemental descendants.
Races of Faerun does suggest, that the servants of elemental deities sometimes produce genasi offspring with humankind.
Even if such an elemental deity is evil- so its servants would really be fiends and ought to produce tiefling descendants. Umberlee, and water genasi, is the example given.