Replicative fading was the degeneration of viable DNA arising from the practice of reiterative cloning. Due to mutations in the DNA sequence, each successive clone had more deleterious conditions and genetic traits, since each generation was cloned from the generation before it. Eventually, this "fading" resulted in the complete inability for new viable clones to be created. The only solution to replicative fading was to introduce DNA from non-cloned individuals, thus reducing the relative concentration 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")