They are also known as accessory chromosomes or supernumerary chromosomes. These chromosomes exhibit lineage specificity. Not all the individuals in a given population possess B chromosomes. For example, only 10-15% of the plant and animal species possess these chromosomes. These chromosomes possess tiny structures and present themselves in multiple copies. Thus, they occur in a few species and may not be so essential. Examples include certain species of wild animals, plants, and fungi. They show the presence of complete heterochromatin. Maize contains euchromatin in addition to the heterochromatin. Some of the species show the presence of metacentric and submetacentric B chromosomes. These chromosomes are also known as conditionally dispensable chromosomes. They mainly become visible in the warm or dry climatic conditions. They get accumulated in the meiotic cell products. The B chromosomes lead to an increased asymmetry in chiasma distribution and suppress the homologous chromosome pairing. They help in reducing multiple pairing in the homologous chromosomes in the allopolyploidy. They impose various effects on the A chromosomes. For example, they increase the crossing over and recombination frequencies. They also lead to an increase in infertility due to an increased number of unpaired chromosomes. The dispensable genetic elements occur in the rye and maize. These B chromosomes lack homology with any member of the basic set of chromosome. These B chromosomes follow the non-Mendelian pattern of inheritance. The number of B chromosomes remains constant in the somatic tissue. It involves increased intraspecific variation in nuclear DNA. The species with large genomes tolerate them. Thus, they exhibit very high rates of polymorphisms.