Genasi are the descendants of both mortal races and elementals. Like tieflings and aasimar, genasi owe their existence to Planescape and "grid-filling", being the inevitable Elemental branch of the Planetouched family tree. Genasi got carried over to 4e, while the aasimar got dumped in favor of a new concept. In older editions, there was one genasi type for each of the Elemental Planes. Genasi of the same elemental type could look wildly different, even if they had the same kind of elemental ancestor. For example, one water genasi might be covered in fish-scales and have webbed hands, a second might look like a humanoid frog, and a third might be a normal human with blue hair.
3.5 added a "half-elemental" template that gave some explanation as to where genasi might have come from, but still, most people chose to ignore them in these editions. Additionally, earth genasi, in a rare move for D&D races, were explicitly noted to come from non-human stock. Dwarvish, to be precise, which only becomes more hilarious the more you think about it. ("When a dwarf loves his mine very, very much..." Bram Rockfucker, etc.)
Unofficial "expansions" to the Genasi family tree came in Dragon Magazine #297 (Paraelemental Planes Genasi) and Quoth the Raven #7, a Ravenloft fansplat that featured an article on Dread Genasi (Genasi of Grave, Mist, Blood and Pyre, the corrupted forms that ordinary elementals got twisted into if summoned there).
It wasn't until 4th edition that genasi got promoted to a full-bore playable race, in the Forgotten Realms Player's Guide. This makes sense because FR has always had a fairly substantial population of them; one of the more exotic countries spent a long time under the rule of Genies, so there are lots of genie-blooded genasi running around. Dragon Magazine #350 even introduced the Azerblood and D'hin, genasi of dwarf/azer and halfling/djinn ancestry.
Genasi got a 5e update in the Elemental Evil campaign, appearing in both the Princes of Elemental Evil adventure book and the "Elemental Evil Player's Companion" web-enhancement. Which can be found [here, if you want to chase it].
Genasi are easily recognizable: though of similar built to humans, they tend to be slightly taller and broader in build. That's not their most distinct feature; their skin is of a most definitely non-human color, ranging from brown to blue, purple, red and pretty much anything you can mention. Their skin is also marked with a pattern of lines called szuldar, with the patterns of blood relatives looking somewhat alike. Some amongst them display their lines with pride, wearing clothing that exposes a good amount of skin. Genasi have no hair, though certain manifestations have crystaline growth on their heads.WotC), over time the genasi comes to control the new manifestation. Nearly all of those who develop an additional manifestation have a clear barrier between the two: though there are some who embrace the strife between their manifestations, and become Elemental Tempests. The most common genasi are those who share a manifestation with the four classical elements: Earth, Fire, Water and Wind. A common fifth manifestation is the Storm, because everybody likes lightning. A darker aspect of the genasi is derived from their association with the Elemental Chaos. Because it touches on the Abyss, sometimes a genasi will be born with a corrupt manifestation, and carry the touch of the Abyss with them for the rest of their lives. These are the Caustic, Cinder, Plague and Void souls, twisted versions of the Water, Fire, Earth and Wind manifestations. Lightning doesn't get one.
In Dark Sun they have different manifestations, namely Ember, Magma, Sand and Sun.
The genasi are a highly emotional race. A desire for change is deep-rooted in the race, to the point that even Lawful Good genasi embrace the idea of change. Genasi society is small due to the nature of the Elemental Chaos, yet highly dynamic. It is rare to find a genasi that has held the same position for several years, or in some cases, even months. You rise and fall in the eyes of a genasi based on merit and ambition, but nothing lasts forever, and you'll quickly need to find something to keep proving yourself. In comparison to other races, genasi settlements know few paupers and less to no permanent poor folk, for a near-impossible ability to feel pessimism and a constant drive to improve one's lot in life quickly restores one's social standing. (as Cruijff would say: "If you're at the bottom there's only one way you can go: up.") The idea of nobility and royalty is foreign to the genasi, as one's position could be undone at any moment. The downside to this view is that the genasi don't look to the future, they only know that it'll be different, embracing it whatever it may be.
As is to be expected from their wild and competitive nature, genasi love competitive sports that focus on the individual, like feats of strength, tests of arms, or in extreme cases, gladiatorial combat. Whereas another master craftsman (for example, a dwarven smith) might make a weapon that is of great quality, a genasi craftsman prefers to tailor-make every item to the needs and wants of a customer, a genasi-made blade is perfect for an individual person. Likewise, genasi works of art are vibrant, bright items that express the wide range of emotions of the artist.
Now the offspring of Genies and Humans, because it's kinda hard to explain how you fucked a rock and the rock gave birth, though Genasi can also be a result from a surge of elemental power from a planar convergence or an eruption from the Inner Planes. Also different from their 4e counterparts as they can grow hair. Genies rarely ever contact their offspring, seeing them as accidents.
Stat-wise: +2 Con, tend towards Neutral alignments, live a little longer than humans, but not much. Majority of bonuses come from their subrace.
Air Genasi get +1 Dex, unlimited breath while not incapacitated, free levitate every long rest.
Earth Genasi get +1 Strength, Earth Walk (Difficult terrain does not apply if made of earth or stone), and Merge With Stone. (free Pass Without Trace once per long rest, with Con as the casting ability)
Fire Genasi get +1 Intelligence, Darkvision (except everything is red, instead of gray), fire resistance, and Reach to the Blaze. (Produce Flame Cantrip known immediately, get Burning Hands once per long rest at 3rd level. (Con as the Spellcasting Ability))
Water Genasi get +1 Wisdom, acid resistance, Amphibious (breather air and water), swim speed of 30 ft, and Call to the Wave. (Free Shape Water cantrip, get Destroy Water at level 3 once per long rest (Con as the Spellcasting Ability))
Needs some work.
|Dungeons & Dragons 4th Edition Races|
|Player's Handbook 1:|| Dragonborn - Dwarf - Eladrin - Elf |
Half-Elf - Halfling - Human - Tiefling
|Player's Handbook 2:||Deva - Gnome - Goliath - Half-Orc - Shifter|
|Player's Handbook 3:||Githzerai - Minotaur - Shardmind - Wilden|
|Monster Manual 1:|| Bugbear - Doppelganger - Githyanki |
Goblin - Hobgoblin - Kobold - Orc
|Monster Manual 2:||Bullywug - Duergar - Kenku|
|Dragon Magazine:||Gnoll - Shadar-Kai|
|Heroes of Shadow:||Revenant - Shade - Vryloka|
|Heroes of the Feywild||Hamadryad - Pixie - Satyr|
|Eberron's Player's Guide:||Changeling - Kalashtar - Warforged|
|The Manual of the Planes:||Bladeling|
|Dark Sun Campaign Setting:||Mul - Thri-kreen|
|Forgotten Realms Player's Guide:||Drow - Genasi|
|Dungeons & Dragons 5th Edition Races|
|Player's Handbook:|| Dragonborn - Drow - Dwarf - Elf - Gnome |
Half-Elf - Half-Orc - Halfling - Human - Tiefling
|Dungeon Master's Guide:||Aasimar - Eladrin|
|Eberron Unearthed Arcana:||Changeling - Shifter - Warforged|
|Elemental Evil Player's Guide:||Aarakocra - Genasi - Goliath - Gnome (Svirfneblin)|
|Princes of the Apocalypse:||Genasi|
| Dwarf (Duergar) - Halfling (Ghostwise) |
Gnome (Svirfneblin) - Tiefling (Variants)
|Waterborne Adventures UA:||Minotaur|
|Gothic Heroes UA:||Revenant|
|That Old Black Magic UA:||Tiefling (Abyssal)|
|Volo's Guide to Monsters:|| Aasimar - Bugbear - Firbolg - Goblin - Goliath |
Hobgoblin - Kenku - Kobold - Lizardfolk
Orc - Tabaxi - Triton - Yuan-Ti Pureblood
|The Races of Pathfinder|
|Player's Handbook:||Dwarf - Elf - Gnome - Half-Elf - Half-Orc - Halfling - Human|
| Aasimar - Catfolk - Changeling - Dhampir - Duergar |
Drow - Fetchling - Gillman - Goblin - Grippli - Hobgoblin
Ifrit - Kitsune - Kobold - Merfolk - Nagaji - Orc - Oread
Ratfolk - Samsaran - Strix - Suli - Svirfneblin - Sylph
Tengu - Tiefling - Undine - Vanara - Vishkanya - Wayang
|Bestiaries||1:|| Aasimar - Drow - Gnoll - Goblin - Hobgoblin |
Kobold - Merfolk - Tengu - Tiefling
|2:||Fetchling - Dhampir - Grippli - Ifrit - Oread - Sylph - Undine|
|3:||Catfolk - Ratfolk - Suli - Vanara - Vishkanya|
|4:|| Changeling - Gathlain - Kasatha - Kitsune - Nagaji |
Samsaran - Trox - Wayang - Wyrwood - Wyvaran
|5:|| Android - Astomoi - Caligni - Deep One Hybrid |
Ghoran - Orang-Pendak - Reptoid - Shabti
|6:||Monkey Goblin - Munavri - Naiad - Rougarou - Yaddithian|
|Inner Sea Races:|| Ghoran - Monkey Goblin - Lashunta - Skinwalker |
Syrinx - Triaxian - Wyrwood - Wyvaran