|Out of universe information|
Wraithkin, also known as The Taken, refers to a group of people (the only ones known are Athosians) whose ancestors were culled but spared from death, and who later possessed extra-sensory abilities. Charin was the only surviving member of the Athosians who remembered stories of the Taken, and eventually passed on the knowledge to Teyla Emmagan.
According to the legend there were a select few individuals who were returned to a planet in the Pegasus galaxy. Many believed it was a gift from the Ancients that their loved ones returned, but many others feared them, as those who returned began to behave strangely, spoke of hearing voices, and some even became violent and murderous. Eventually, the villagers of this world turned on the Taken, killing all but a few.
The ones that had survived were ostracized from the community, but left through the Stargate and passed on the "gift" to their children. The only known planet where Wraithkin settled and mixed with the inhabitants is Athos. However, presently very few Athosians have the abilities of the Taken.
Teyla Emmagan and the First Atlantis Reconnaissance Team discovered, in 2005, that it was because of a Wraith who was studying ways to enhance the feeding process so that humans would essentially become "tastier" to them. He was doing it against the wishes of the other Wraith, which was why he was doing it in secret. Later the Wraith conducting the experiments realized that he was enhancing his enemy with Wraith abilities and eventually gave up on the experiment. The Wraithkin all possessed the ability to sense the Wraith's presence and Teyla, once she learned she could do it, and with some help from hypnosis, figured out how to use her abilities to telepathically link to various Wraith. Teyla was also vulnerable to being possessed by Wraith, but in one instance, with her telepathy bolstered by her unborn child, she was able to take control of a Wraith Queen's body instead. Teyla also discovered the ability to control Wraith technology such as a Hive ship or presumably Wraith cruiser which required Wraith telepathic controls to work. However, when not pregnant with her son who was also Wraithkin, Teyla found it difficult and exhausting to pilot such ships. (SGA: "The Gift", "Halcyon", "Misbegotten", "Spoils of War")
After receiving training from Guide when she posed as his Queen, Teyla became more adept at using her mental powers to the point of having the strength of a Wraith Queen. She grew capable of piloting Wraith ships and killing Wraith with her mind by stopping their body processes. She later understands that before she lacked experience and feared her abilities so she didn't embrace them. With training and discipline, Teyla was able to use the powers granted to her to kill Queen Death, a powerful Wraith Queen. She was also able to access the same Wraith genetic memory that Wraith Queens could. (SGA: "Legacy")
A Wraith named Thorn later explained to Queen Waterlight how the Wraithkin came to be: the scientist who created them mingled his genes with his test subjects, partially transforming them before releasing them to breed. As this was against the old ways of the Wraith, his own sister stopped him, closing his testing facility and destroying his notes so that no one else could follow in his footsteps. However, by that point, the Wraithkin had spread out to a dozen worlds with no way to seek them out or identify them. The Wraith scientist tried again so his sister was forced to kill him. According to Thorn, the Wraith believe that within five to six generations, the Wraith blood in the Wraithkin would become so dilute that they would possess no abilities and so the Wraith no longer saw the Wraithkin as anything more than an interesting matter. (SGA: "Inheritors")
In 2010, the Tau'ri encountered a Wraith worshipper amongst the Travelers named Deekas who revealed himself to be Wraithkin. Unlike the Athosians, his people didn't embrace Wraithkin and instead offered them up as tributes to the Wraith. Deekas was one such Wraithkin who was treated like this. (SGA: "The Third Path")