This is very poorly written. :(

> DNA sequencing and analysis of duplicated mice revealed no differences between early and later generations of clones. But that began to change after generation 27, when genetic abnormalities began to negatively affect fertility.

If there were "no differences" between early and later generations of clones, then what "abnormalities" are they talking about?

No differences in the DNA sequences for N iterations ... and yet physical and genetic abnormalities eventually appeared after N+1.

The suggestion then is that other information about the iteration of copies of copies is being passed, from the article:

  There may have been several causes behind the abnormalities that arose in later generations.

  {various potential causes discussed}

Limitations of serial cloning in mammals - https://www.nature.com/articles/s41467-026-69765-7