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(accessory chromosome, supernumerary chromosome) Any of a number of small chromosomes present in some organisms in addition to the fixed number of stable chromosomes characteristic of the species (the A-chromosomes). They are of widespread oc-currence and have been reported in over 80 genera and 20 families of plants. The numbers found may differ between members of a species and even within a single individual, with gametic nuclei often having larger numbers. B-chromosomes are composed largely of heterochromatin and in many cases appear to have little effect. However when present in large numbers they may be deleterious.
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