Title of post apocalytic novel, sterility, clones, pennsylvania?

I read this novel that I believe was written around 1975. Something - I think it was ecological - has resulted in the almost universal sterility. A (rich) family has created an enclave somewhere in the mountains, probably in Pennsylvania. They have developed quite advanced cloning techniques and the clones are fertile. But there's a problem - around the 4th generation, some genetic problems arise.

I thought the title was "Earth Abides", but now I know that can't be it!

Help me!