Q.What are B - chromosomes ?

Chromosomal mutations (aberrations) alter chromosomes on a scale spanning many genes at once, often visible microscopically, unlike gene mutations. They fall into two groups. Numerical aberrations (ploidy changes) are aneuploidy, a gain/loss of individual chromosomes within an otherwise diploid set -- hyperploidy (trisomy 2n+1, double trisomy 2n+1+1, tetrasomy 2n+2, pentasomy 2n+3) and hypoploidy (monosomy 2n-1, double monosomy 2n-1-1, nullisomy 2n-2, usually lethal) -- or euploidy, a change in the number of complete chromosome sets: monoploidy (x), diploidy (2n), and polyploidy (more than 2 sets), split into autopolyploidy (all sets from one species, e.g. autotriploid banana, autotetraploid potato/rye/coffee) and allopolyploidy (sets from different species, fertility restored by colchicine-induced chromosome doubling after a sterile hybrid stage, e.g. Karpechenko's Raphanobrassica from radish x cabbage, and Triticale, the first man-made cereal, from wheat x rye, combining wheat's protein content with rye's lysine content). Structural aberrations rearrange gene order/copy number within or between chromosomes after a chromosome break (from radiation or chemicals): deletion/deficiency (terminal or intercalary loss of a segment), duplication/repeat (tandem, reverse …