As we've mentioned, scientists thought that chromosomes undergo irreversible maturation as the body develops, making them unable to revert to the primitive stage of embryonic chromosomes. Thus, nerve cells don't make hair or skin proteins, even though they have the genetic instructions to do so. Dolly is apparently baa-ing proof that this is not a problem. Chromosomes can apparently revert to a primitive stage. Chromosomes also change through time when markers called telomeres are added to their ends.
Telomeres are apparently a way of counting and controlling how many times a cell can reproduce.
That could mean that Dolly will die young as her chromosomes follow their programming and stop reproducing. Then again, maybe not. Most of these breaks are repaired by normal cell processes. Usually, the rest don't cause problems because they are on genes that function only during development, and not after maturity. For that very reason, nobody knows exactly how chromosomes change as an animal grows up.