B chromosome

B chromosomes are chromosomes which are not needed for the life of a species. They are also called 'supernumerary' or 'accessory chromosomes'.
In addition to normal chromosomes (the karyotype), many species have B chromosomes. Wild populations of many animal, plant, and fungi species have B chromosomes. These chromosomes are not essential for their life, and are lacking in many individuals. So a population might have individuals with 0, 1, 2, 3 (etc.) supernumeraries.
Most B chromosomes are mainly or entirely heterochromatic, and so do not code for anything. Some, such as the B chromosomes of maize, contain sizeable euchromatic segments, and therefore do code for some products.
It seems unlikely that supernumeraries would survive in a species unless there was some positive advantage. In a few cases this has been identified. For instance, the British grasshopper "Myrmeleotettix maculatus" has B chromosomes with a satellite DNA. They occur in warm, dry environments, and are scarce or absent in humid, cooler places.
It is thought that B chromosomes are parasitic elements like selfish DNA. Various scenarios have been suggested as to how they arose.