B-Chromosomes

 

 

B-chromosomes are supernumerary chromosomes that may be absent or present as extra chromosomes in various species, both plants and animals.

Features:

1. Dispensable Nature: B-chromosomes are not found in all individuals of a species and may be absent in some cells of individuals that possess them.
2. Non-Homologous: They are non-homologous with the basic set of chromosomes (A-chromosomes).
3. Non-Mendelian Inheritance: B-chromosomes do not follow Mendelian patterns of inheritance.
4. Size: They are generally smaller than A-chromosomes.
5. Heterochromatin Distribution: They have a unique pattern of heterochromatin distribution and are typically telocentric in nature.
6. Genetic Impact: In general, B-chromosomes are genetically inert or have minimal effects on the organism.
7. Effect on Vigor and Fertility: When present in high numbers, B-chromosomes can suppress the vigor and fertility of the organism.
8. Occurrence: They are often found in outbreeding species.
9. Tissue Specificity: Sometimes B-chromosomes are restricted to certain tissues, such as aerial parts of plants and absent in roots (e.g., Aegilops speltoides and Aegilops mutica).
10. Pollen and Seed Effects: They can affect pollen fertility and seed set.
11. Phenotypic Effects: B-chromosomes can delay flowering time and adversely affect other traits such as plant height and number of tillers.