Supernumerary Chromosomes

In some species chromosomes have been found which are in addition to the normal autosomes and heterosomes. These chromosomes are genetically unnecessary and have been called supernumerary chromosomes, accessory chromosomes or B chromosomes. Supernumerary chromosomes are not homologous with any of the normal chromosomes, and they do not form synapses with the latter. Supernumerary chromosomes are not present in all the individuals of a species but are confined to a few individuals.

Supernumerary chromosomes do not usually have any effect on the phenotype and hence are genetically unnecessary. In some plants, however, supernumerary chromosomes result in decreased vigor.

Usually each nucleus has one or two supernumerary chromosomes. In Tradescantia edwardsiana there are 5 supernumerary chromosomes in addition to the twelve somatic chromosomes. Supernumerary chromosomes may be eliminated from certain tissues of organs during embryogenesis, but may persist in others.



Supernumerary chromosomes are believed to be derived from autosomes. They have retained the centromere and the heterochromatic genetically insert parts of the arms on either side of the centromere. The distal euchromatic parts of the arms containing genes have, however, been lost.




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