Replicative fading

From Memory Alpha, the free Star Trek reference.

Replicative fading is the degeneration of viable DNA arising from the practice of reiterative cloning. Due to mutations in the DNA sequence, cloning becomes more and more difficult with subsequent generations. Eventually, this "fading" results in the complete inability for new clones to be conceived. The only solution to replicative fading is to introduce DNA from non-cloned individuals, thus reducing the number of harmful mutations.

By 2365, the colony on the planet Mariposa was populated entirely by clones from five individuals and replicative fading was threatening their very existence. The colonists were encouraged to interbreed with a nearby colony even though they were socially incompatible. (TNG: "Up The Long Ladder")