B-Chromosomes are a particular kind of supernumerary chromosomes, that may or may not be found in an organism as extra chromosomes over and above the standard diploid or polyploid chromosome complement (see Fig. 6.18). The standard complement consists of chromosomes described as A-chromosomes, inciting sex chromosomes, if any. The B-chromosomes are found in the natural populations of many plant and animal species and are recognized on the basis of their following characteristics : (a) they are dispensable; are not found in all individuals of a species and may not be found in all cells of an individual organism; (b) they are not homologous with any of the basic A chromosomes; (c) their inheritance is non-Mendelian, sometimes due to non-disjunction during pollen mitosis (as in some plants); (d) they are usually smaller than A-chromosomes and have their own unique pattern of heterochromatin distribution; (e) in general, they are genetically inert, but may rarely organize nucleoli and carry functional genetic material; (f) when present in high number, they suppress vigour and fertility; (g) their origin and function are unknown.

B-chromosome in plants have been reported in more than 1000 species distributed over bryophytes (including mosses), ferns, gymnosperms and angiosperms (both monocotyledons and dicotyledons). Mostly, they are found in out-breeders (cross pollinated species). In the family Gramineae, they are found in 200 species including cereals, and forage grasses belonging to genera Agrostis, Anthoxanthum, Avena, Bromus, Calamogrostis, Dactyiis, Deschampsia, Festuca, Koeleria, Lolium, Pennisetum, Phleum, Secale, Sorghum and Zea. These B-chromosome are often absent in well adapted agronomic strains, and are perhaps eliminated due to selection. The B-chromosomes may sometimes be restricted only to aerial parts (absent in roots e.g. Aegilops speltoidas, Ae. mutica) and may also be eliminated during meiosis due to their division at anaphase I followed by elimination at anaphase II. B chromosomes may also increase in number due to a drive involving non- disjunction during pollen mitosis, thus leading to the production of plants with two B-chromosomes in the progeny OB ♀ x 1B ♂.