Jag tror att du kanske kommer ihåg Hur sent söta fågeln sjöng av Kate Wilhelm.
Likheter: Post-apokalyptisk, sterilitet, rik familj, kloning, enklav.
Olikhet: Enklaven är i Virginia, inte Pennsylvania.
Det vann Hugo Award för bästa roman 1977, så det ligger nära din uppskattning av 1975.
Från dammjackets kopia:
It is the story of the Sumners of Virginia, who have ruled over a lush valley along the Shenandoah for generations. But the Sumner's land, their money, their influence do not make them immune to the war and pestilence that destroy the rest of the earth. Through the foresight of the elder Sumners, they create a scientific research center that provides them with a means of survival. But as the Sumner men become sterile, the women barren, young David Sumner, his cousin Celia, and the other Sumners become victimized by an army of look-alike, think-alike clones who are able to perpetuate their Sumner bodies - but not their humanity. What hope there is for a real, human future must lie with the outcasts of the new society.
Skyddsskott från ISFB -