Chromosomes don't really have a common nickname. They are often referred to as:

* Chromatin threads when they are in their uncoiled, less condensed form.

* Sister chromatids when they are duplicated and joined together.

* Karyotype when referring to the complete set of chromosomes in an organism.

While some might use informal terms like "chromo" or "chromie" jokingly, these are not widely accepted nicknames.