This chapter is a continuation of the discussion of unusual chromosome types. As mentioned before, the term “unusual” in this connection is a relative term. We very often have a certain concept of things, and whatever deviates from this concept we call “unusual”. Because ring-chromosomes, telocentric chromosomes, isochromosomes, or B chromosomes differ from the majority of chromosomes in humans, animals and plants, they are considered unusual. But apparently, in some instances, such chromosomes fulfill a need that cannot be met by any other chromosome type. There is no obvious connection between these four chromosome types except that they all deviate in some way or another from the prototype as described in Chapters 2 and 3.