B chromosome

B chromosomes are small supernumerary chromosomes found in many eukaryotic nuclei that are believed to be largely non-coding and probably parasitic on the host genome.

B chromosomes play an important role on normal A chromosomes. The B chromosomes suppress homologous pairing which reduces multiple pairing between homologous chromosomes in allopolyploids. Bivalent pairing is ensured by a gene on chromosome 5 of the B genome Phlocus. The B chromosomes also have the following effects on A chromosomes:


 * increases asymmety chiasma distribution
 * increases crossing over and recombination frequencies: increases variation
 * cause increased unpaired chromosomes: infertility

B chromosomes have tendency to accumulate in meiotic cell products resulting in an increase of B number over generations. However this effect is counterbalanced for selection against infertility.