Mutants

"Mutation: it is the key to our evolution. It has enabled us to evolve from a single-celled organism into the dominant species on the planet. This process is slow, and normally taking thousands and thousands of years. But every few hundred millennia, evolution leaps forward."

- Jean Grey

Mutants, also known as Homo superiors, are individuals who possess a genetic trait called the "X-Gene" that allows them to naturally develop superhuman powers and abilities and sometimes affecting their appearance.

Pre-Discovery
The first mutant appeared during ancient Egypt thousands of years ago.

X-Men Origins: Wolverine
James Howlett and Victor Creed whom fought in important wars of American history were the next earliest known mutants. By the time of World War II, mutants became a more widespread phenomenon though humanity at large continued ignorant of their existence.

X-Men: First Class
It was not until 1960's when the existence of mutantkind was revealed during the Cuba Missile Crisis, what orchestrated by Sebastian Shaw who sought to destroy baseline humans so that mutants could rule the Earth, with Shaw as their leader. His plans were thwarted by a group of mutant misfits, whom used their unique gifts to prevent a nuclear war. However their heroic actions exposed mutants to the world in irrevocable ways.

X2: X-Men United
Colonel William Stryker, Jr. sought to rid the world of mutantkind by using a combination of his son Jason and Charles Xavier's powers with Cerebro to mentally kill all mutants in the world. His plan nearly backfired at him and not only nearly caused homo sapiens' extermination but also a full-out war between mutants and humans.

X-Men: The Last Stand
The billionaire businessman Warren Worthington II sought to cure his son of his "ailment" and used his corporation to create a mutant cure, using a mutant child as source. The cure was supposed to be used to cure mutants and make them human again but it was also used as a weapon by USA military in case dangerous mutants posed a threat to lives.

Learning of this mutant supremacist Magneto lead an army to the headquarters of Worthington Labs, in San Francisco, to destroy it before it was used to exterminate mutantkind.

X-Men: Days of Future Past
In 1973, the Sentinel Program was used after Bolivar Trask was killed by Mystique. Later on, with her DNA, they create the Sentinel Mark V, a Sentinel that can adapt to it's environment. With these, the Sentinels take over the world, exterminating mutants, those with the mutant gene, and those that supported mutant resistance, thus leaving the worst of mankind left. Few survive. This however is fixed when Wolverine is sent back in time to stop Mystique.

X-Men: Apocalypse
Ten years in later in 1983 thanks to Mystique heroic actions in Washington D.C Mutants have become almost universally accepted. However when the worlds first mutant En Sabah Nur was awaken he quickly began a plan to cleanse the world of the “weak” who he saw as ruling the Earth now. Recruiting a new set of Horsemen consisting of a young Storm, and grieving Magneto, a lost Angel, and an impressed Psylocke. Apocalyse plan almost succeeded but fortunately he was stopped by the reformed X-Men. However Apocalypse actions made Mutants once again controversial to the public.

X-Men: Dark Phoenix
To be added

Deadpool
In the present alternate timeline things such as genetic manipulation, torture methods, and near-death experiences were found to be able to cause an artificial mutation in Mutants who had any inactivate X-Genes in them. Known as the Workshop the programs goal was to unlock a persons mutant powers then take away their will and sale them off to the highest bidder. However when one of “volunteers” Wade Wilson (who had come to the program in hopes they fix his cancer) managed to escape he went on a systematic rampage for the Workshops leader Ajax. When Ajax managed to kidnapp Wade’s girlfriend Vanessa he sought the help of Colussus and Negasonic Teenage Warhead to help rescue her. With their help Deadpool was able to kill Ajax ( finally getting his revenge) and reunite with Vanessa. Mutant experimentation was also done on mutant children as well. Five mutants in particular Illyana Rasputin, Samuel Guthrie,Rahne Sinclair, Roberto da Costa,and Danielle Moonstar were trapped inside a hospital and were forced to escape from not only it but a another malevolent presence inside the facility.

The Gifted Episode 1.01: eXposed
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The Gifted Episode 1.02: rX
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The Gifted Episode 1.03: eXodus
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The Gifted Episode 1.04: eXit strategy
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Deadpool 2
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X-Force
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Gambit
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Logan
In the early 21st century the scientific company know as Transigen was able to almost eliminate the X-Gene by making a genetically modified corn syrup which was then planted in foods and drinks that would be consumed by the larger populace. With this the number of mutant births declined significantly until the point in 2029 in which no new mutant birth was reported to have happened in the last 25 years.

In the year 2028, Charles Xavier became stricken with Alzheimers and as a result unleashed a psychic seizure that not only injured 600 civilians but also killed several members of the X-Men. By 2029 the X-Men had been all but disbanded leaving only Xavier and Logan who became Charles caretaker but was also suffering from the fact that his healing factor was slowly fading.

Genetics
Mutants are born with a genetic trait called the "X-Gene", which alters the physiological structure of the individuals granting them superhuman powers and abilities, some of these powers the result of alteration of their appearance. These alterations, called mutation, is seemingly random and its rare (but not impossible) for two mutants to share the same mutation.

Trivia

 * In the comics, mutants were created due to a race of beings known as the celestial experimenting on humans in the ancient past, giving them the X-Gene.
 * In the comics, Class 5 mutants, are known as Omega-level mutants; they are the strongest and most powerful of the mutant race.