I'm not very knowledgeable about mammalian chromosome, but some plants can
have variable numbers of extra, small chromosome. In the Allium species
(chives, garlic, etc) these are called B chromosomes or supernumary
Some mammals also have B chromosomes, but I can't remember any names
as I only studied chives in my project at York.
In chives these B chromosomes can be passed down for several generations.
As far as I know, no-one knows where these B chromosomes come from, though
someone may have found out in the last 3 years.
You could try a literature search on "B chromosomes" or "supernumary
chromosomes" . You could also try papers by S. Bougourd, who works on
Allium species at York.