Fallen Angels of Atlantis and Archaic DNA

Having once been a privileged and favored people, the Atlanteans became corrupt and degraded, according to Plato, by interbreeding with mortals and diluting their noble seed which allegedly descended from the gods. So what did Plato mean by mortal admixture, when he said that the Atlantean divine portion began to fade away and became too diluted, too often, by the mortal admixture?