Mutant (Marvel Comics)

In American comic books published by Marvel Comics, a mutant is a human being that possesses a genetic trait called the X-gene. It causes the mutant to develop superhuman powers that manifest at puberty. Human mutants are sometimes referred to as a human subspecies Homo sapiens superior, or simply Homo superior. Mutants are the evolutionary progeny of "Homo sapiens", and are generally assumed to be the next stage in human evolution. The accuracy of this is the subject of much debate in the Marvel Universe.
Unlike Marvel's mutates, which are characters who develop their powers only after exposure to outside stimuli or energies (such as the Hulk, Spider-Man, the Fantastic Four, Absorbing Man and Captain Marvel), mutants have actual genetic mutations.
Background.
A March 1952 story in "Amazing Detective Cases" #11 called "The Weird Woman" tells of a woman describing herself as a mutant who seeks a similarly superhuman mate.
Roger Carstairs, a mutant who can create illusions, is shown in "Man Comics" #28, dated September 1953.
A character with superhuman powers, born from a radiation-exposed parent, was seen in "The Man with the Atomic Brain!" in "Journey into Mystery" #52 in May 1959; although not specifically called a "mutant", his origin is consistent with one.
A little-known story in "Tales of Suspense" #6 (November 1959) titled "The Mutants and Me!" was one of the first Marvel (then known as Atlas) stories to feature a named "mutant".
Tad Carter, a mutant with telekinetic powers, is shown in Amazing Adult Fantasy #14, dated July 1962.
The modern concept of mutants as an independent subspecies was created and utilized by Marvel editor/writer Stan Lee in the early 1960s, as a means to create a large number of superheroes and supervillains without having to think of a separate origin for each one. As part of the concept, Lee decided that these mutant teenagers should, like ordinary ones, attend school in order to better cope with the world, in this case Xavier's School for Gifted Youngsters. These mutants first appeared in the superhero series "X-Men", which debuted in 1963. Marvel later introduced several additional mutant superhero teams, including The New Mutants, X-Factor, Excalibur, X-Force, and Generation X.
Officially, Namor the Sub-Mariner is considered the first mutant superhero whom Marvel Comics ever published, debuting in 1939. However, Namor was not actually described as a mutant until "Fantastic Four Annual" #1, decades after his first appearance. The same is true of Toro, partner of the android Human Torch introduced in 1940.
Omega-level mutants.
An Omega-level mutant is one with the most powerful genetic potential of their mutant abilities. The term was first seen in the 1986 issue "Uncanny X-Men" #208 as "Class Omega", but was completely unexplained beyond the obvious implication of it referring to an exceptional level of power. The term was not seen again until the 2001 limited series "X-Men Forever". For a time, no firm definition was offered in the comics, leading to several conflicting opinions and debates as to who or what qualified as Omega-level. In July 2019, Marvel provided an official definition in the X-Men relaunch starting in "House of X" by Jonathan Hickman [emphasis in original]:
The following are the known Omega-level mutants as of the "House of X" relaunch: Jamie Braddock, Iceman, Elixir, Jean Grey, Legion, Magneto, Proteus, Mister M, Storm, Exodus, Quentin Quire, Vulcan, Hope Summers. 
Franklin Richards was recently considered beyond Omega-level, however it has since been revealed that he is not actually a mutant, instead he unconsciously altered his DNA when he was a child to make it appear that he had the X-Gene to make himself special. Due to this revelation, he is no longer welcome on Krakoa. 
Before and after the "X of Swords" event, a number of new Omega-level mutants were introduced from the island of Arakko, the other half of Krakoa that until recently had been trapped in the dark dimension of Amenth. The wife of Apocalypse, Genesis, is Omega-level and rumored to be, if not more so, as powerful than her husband. Their four children, the original Horsemen of Apocalypse, are also Omega-level. The equivalent of the Quiet Council of Krakoa, the Great Ring of Arakko, is comprised only of Omega-level mutants. The current members are: Isca the Unbeaten, Idyll, Tarn the Uncaring, Ora Serrata the Witness, Stulgid, Lodus Logos, Lactuca, Sobunar and Redroot. There are a rumored 3 more members to the Ring to fill the 'Night' seats, but they are yet to be revealed.
"Homo superior superior".
Introduced in Chris Claremont's "X-Treme X-Men", a character known as Vargas claims to be humanity's natural response to mutants. Vargas was born at the epitome of peak physical skill, having superhuman levels of strength, speed, reflexes, agility, stamina, and durability. Vargas also seems to be immune to various mutant abilities (such as Rogue's absorption and Psylocke's telekinetic blast).
Externals.
Created by Rob Liefeld, Externals are immortal mutants whose powers have allowed them to exist for centuries. Eventually, most of the Externals are killed by Selene. Gideon, Selene, and Apocalypse are examples of Externals.
Cheyarafim and Neyaphem.
Cheyarafim and Neyaphem first appear in "Uncanny X-Men" #429. According to the character Azazel, the Cheyarafim are a group of angel-like mutants who were the traditional enemies of the Neyaphem, a demonic-looking group of mutants who lived in Biblical times. The Cheyarafim were fanatics who had a strict, absolutist view of morality which led them into conflict with the Neyaphem. This escalated into a holy war, causing the Neyaphem to be exiled into an alternate dimension. What happened to the Cheyarafim after this has not been revealed.
The X-Man Angel is said to be descended from Cheyarafim, while Nightcrawler is supposedly the son of a Neyaphem, Azazel.
Dominant Species/lupine.
Maximus Lobo claims to be a part of a mutant sub-species of feral, wolf-like mutants, whom he calls the Dominant Species. He later tries to recruit Wolf Cub into his ranks, to no avail. A few years later, another mutant, Romulus, claims that some human mutants evolved from canines instead of primates. Mutants who are a part of this group include Romulus, Wolverine, Daken, Sabretooth, Wolfsbane, Wild Child, Thornn, Feral, and Wolf Cub, with X-23 and the Native as other likely candidates. These groups appear to be one and the same.
Changelings.
Introduced in the second series of "X-Factor", a changeling is a mutant whose powers manifest at birth. Jamie Madrox and Damian Tryp are examples of this sub-class.
Extraterrestrial mutants.
Humans are not the only species to have mutant subspecies. Ariel, Longshot, Cerise, Ultra Girl, and Warlock are examples of mutant aliens.
Chimeras.
In the pages of "House of X and Powers of X," the Chimeras are genetically-altered humanoid mutants who are combined from the DNA of past mutants so that they would have combinations of their power set and also propagate the mutant population. Third generation Chimeras have a 10% failure rate making them unable to be warriors. Fourth generation Chimeras have a corrupted hive mind. They were more common in Moira MacTaggert's ninth life where they were created in Mister Sinister's Breeding Pits on Mars. Examples of these Chimeras are Cardinal (who has the genetic template of Francis Fanny, Nightcrawler, and Rachel Summers), North (who has the genetic template of Emma Frost and Polaris), and Rasputin IV (who has the genetic template of Colossus, Kitty Pryde, Quentin Quire, Unus the Untouchable, and X-23).
Mutants as metaphor.
As a fictional oppressed minority, mutants are often used as extended metaphors for real-world people and situations. In 1982, "X-Men" writer Chris Claremont said, "[mutants] are hated, feared and despised collectively by humanity for no other reason than that they are mutants. So what we have here, intended or not, is a book that is about racism, bigotry and prejudice."
Danny Fingeroth writes extensively in his book "Superman on the Couch" about the appeal of mutants and their meaning to society:
An obvious parallel between homosexuality and mutation is drawn in the feature film "X2", where Iceman's mother asks, "Have you tried "not" being a mutant?" This question (or various forms thereof) is common among parents who find out their children are gay. In the 2011 film "", Hank McCoy (later known as Beast), upon being outed to a colleague as a mutant, responds, "You didn't ask, so I didn't tell."
In his article "Super Heroes, a Modern Mythology", Richard Reynolds writes:
Other versions.
Earth X.
Within the "Earth X" universe, the powers of the vast majority of Marvel's human superheroes were revealed to have been the result of genetic manipulation by the Celestials millions of years in the past.
Ultimate Marvel.
In the Ultimate Marvel universe within the pages of the "Ultimate Origins" #1, it is revealed that super-powered "mutants" were artificially created via genetic modification by the Weapon X program in a laboratory in Alberta, Canada in October 1943. The project was an attempt to produce a supersoldier, inspired by the existence of Captain America. James Howlett was the first individual to be so modified. At some later point, possibly during a confrontation between Magneto and his parents, the mutant trigger was released into the environment worldwide, leading to the appearance of mutants in the general population. Following the events of the "Ultimatum" storyline, information concerning the origins of mutancy was made public and steps were taken in the US to make being a mutant illegal. While the move apparently has majority support among the non-mutant population, a vocal minority has voiced concern that it will lead to witch-hunts and genocide.