The Chosen was the name given by the people of Dalera to those who possessed the Ancient Technology Activation gene. Those on the planet itself were the offspring of Dalera and her Human mate. The Chosen were the leaders of the planet who used the gene to activate the Shield of Dalera as their planet's defensive system against the Wraith.
By the time that the Atlantis Expedition made contact with Dalera, most of the Chosen had died out due to their arrogant view of themselves causing them to seek potential partners only among their fellow Chosen, resulting in gradually-worsening problems caused by inbreeding. However, although the expedition's attempt to offer the gene therapy failed to serve as a compromise, it was soon discovered that one of the younger Chosen had been discreetly carrying out his own 'breeding program', sneaking out of the citadel to engage in various affairs with the lower classes, allowing for a new generation of chosen to be born in time to defend the city against a larger Culling. (SGA: "The Chosen")