Genetic engineering is a fascinating subject in Star Trek history and it links Star Trek: The Original Series' Khan Noonien Singh (Ricardo Montalban) with the creation of Star Trek: Deep Space Nine's Dr. Julian Bashir (Alexander Siddig). Due to the tragedies in Earth's 20th-century history and the crimes of Augments like Khan, genetic engineering is illegal in the United Federation of Planets and this jeopardized Dr. Bashir's career in DS9 season 5 when his true nature was discovered.
In Star Trek canon, advances in DNA resequencing in the late 20th century led to the Eugenics Wars from 1992-1996. Dozens of genetically engineered "supermen" rose up and conquered over 40 nations, waging conflicts with ordinary humans and each other. The Augments, as they were known, were stronger, faster, and more intelligent — but major defects in their genomes made them arrogant, belligerent, aggressive, and power-hungry. One of the scientists who created the Augments theorized that "superior ability breeds superior ambition." The worst of the tyrants was Khan Singh, who at one point, controlled 1/4 of the Earth's population. When the Augments were finally defeated, Khan and 84 of his genetically-engineered followers fled the planet aboard the S.S. Botany Bay. In 2267, Captain James T. Kirk's (William Shatner) U.S.S. Enterprise encountered Khan and his people; the warlord later returned in 2285 to steal the Genesis Device and take revenge on Kirk in Star Trek II: The Wrath of Khan.
The Star Trek: Deep Space Nine season 5 episode "Doctor Bashir, I Presume?" revealed that the station's Chief Medical Officer was also the subject of DNA resequencing, a secret he and his parents kept hidden for almost 25 years. At six years old, Jules Bashir was developmentally far behind other children of his age. His parents, who worried about their son's future, took him to a hospital on Adigeon Prime, where he underwent genetic engineering treatments known as "accelerated critical neural pathway formation." Jules' IQ jumped 5 points every day for two weeks, and every aspect of his physical conditioning was enhanced. Afterward, the Bashirs returned to Earth, moved to a new city, and Julian, as he was now known, embarked on a successful academic career that led to his graduating second in his class at Starfleet Medical before taking his post on DS9.
Dr. Bashir's genetic enhancements were accidentally discovered by Chief Miles O'Brien (Colm Meaney) and Dr. Lewis Zimmerman (Robert Picardo) when Bashir was selected as the basis for Starfleet's Long-term Medical Hologram (LMH). Bashir was prepared to resign from Starfleet because of his lifelong deception, but instead, his father Richard (Brian George) turned himself in and served 2-years at a minimum-security penal colony in New Zealand for violating the Federation's ban on genetic engineering.
The Federation's laws about genetic engineering are because the Eugenics Wars have never been forgotten, even in the 24th century. The Enterprise's 23rd-century battles with Khan also served as a bleak reminder of the dangers of the Augments. As Rear Admiral Bennett (J. Patrick McCormack), Starfleet's Judge Advocate General, explained to the Bashirs:
200 years ago we tried to improve the species through DNA resequencing. And what did we get for our trouble? The Eugenics Wars. For every Julian Bashir that can be created there's a Khan Singh waiting in the wings — a superhuman whose ambition and thirst for power have been enhanced along with his intellect. The law against genetic engineering provides a firewall against such men.
However, Dr. Bashir is living proof that genetic engineering isn't a necessarily direct pathway to creating a psychotic villain. As Chief O'Brien reminded his best friend, "Genetic recoding can't give you ambition or a personality or compassion or any of the things that make a person truly human." The good person Julian is at his core wasn't altered by his DNA resequencing, it just allowed the best parts of him to shine brighter.
Yet Julian may be the rare exception to the rule; the genetically-engineered subjects he worked with later in DS9, unfortunately, proved to be dangerous in spite of their intentions. Star Trek: Enterprise introduced 22nd-century Augments created by Dr. Arik Soong (Brent Spiner), and also Klingon Augments, but they ended up more as reflections of Khan rather than Star Trek: Deep Space Nine's noble Dr. Bashir.