B-chromosomes are extra chromosomes found in some species. They are considered to be non-essential or accessory chromosomes, because they do not seem to affect the phenotype of the organism and may have arisen as a result of mutation. They are made up of genetic material that is highly similar to, or identical to, material found on other chromosomes. However, they can contain unique sequences, so they can still give rise to altered phenotypes.

B-chromosomes do not typically contain genetic information that is necessary for the organism to survive and reproduce, although in rare cases they can affect fertility in some organisms.