B chromosomes, also known as supernumerary or accessory chromosomes, are extra chromosomes found in addition to the standard 'A' chromosomes in a plant's genome. They are generally non-essential for survival.

Structure:
B chromosomes are typically smaller than A chromosomes and often appear heterochromatic (densely packed, genetically inactive), rich in repetitive DNA. They exhibit high variability in size, shape, and gene content, even within the same species. While some minor genes might be present, they generally lack major functional genes vital for basic metabolism, distinguishing them from duplicated A chromosomes. Their origin is often obscure, likely fragments or remnants of A chromosomes.

Behaviour:
The behaviour of B chromosomes is characterized by non-Mendelian inheritance and meiotic/mitotic irregularities:

1. Irregular Segregation: Unlike A chromosomes, B chromosomes do not pair regularly with A chromosomes or each other during meiosis. This leads to irregular segregation, often preferential transmission to one pole or random loss.

2. Non-Disjunction: They frequently undergo non-disjunction at meiosis II or even mitosis (e.g., in germline tissues), increasing their number in subsequent generations or specific plant parts.

3. Phenotypic Effects: In small numbers, B chromosomes usually have little discernible phenotypic effect. However, in large numbers, they can cause deleterious effects like reduced vigor, decreased fertility, or physiological abnormalities, due to gene dosage imbalance or increased metabolic load.

4. Population Dynamics: Their presence and number in a population result from a balance between their accumulation mechanisms (e.g., preferential transmission) and any fitness costs they impose.

Examples include maize and rye. B chromosomes represent a fascinating aspect of genome plasticity.