B chromosomes

Definition

Accessory chromosomes that occur in addition to the standard A chromosome complement in a population. Unlike essential A chromosomes, B chromosomes are non-essential for survival, vary in number among individuals (0, 1, 2, or more per cell), and typically lack functional genes critical for development. They persist through meiotic drive, mitotic instability, or other transmission advantages rather than through beneficial effects on fitness. B chromosomes are structurally and behaviorally distinct from supernumerary copies of autosomes or sex chromosomes seen in aneuploidies such as trisomies.

Etymology

From the alphabetical distinction between essential 'A' chromosomes and accessory 'B' chromosomes; term established in cytogenetics to contrast the standard karyotype with dispensable extra elements.

Example

In the grasshopper *Myrmeleotettix maculatus*, B chromosome number varies geographically: Scottish populations carry up to three small, heterochromatic B chromosomes, while southern English populations often lack them entirely, with no apparent effect on viability or fecundity.

Synonyms

• accessory chromosomes
• supernumerary chromosomes

Related Terms

• A chromosomes
• karyotype
• aneuploidy
• heterochromatin
• Meiotic drive
• chromosomal polymorphism

Usage Notes

The plural form is standard in scientific discourse because B chromosomes are defined by their variable presence across populations and individuals. Specialists reserve 'B chromosome' (singular) for discussing the structural or functional properties of a single element. The term excludes marker chromosomes associated with disease, extra copies of standard chromosomes in trisomies, and mitochondrial or plastid genomes. In insects, B chromosomes often show tissue-specific mitotic instability, leading to mosaicism within an individual.