B chromosome

In general "we may regard supernumeraries as a very special category of genetic polymorphism which, because of manifold types of accumulation mechanisms, does not obey the ordinary Mendelian laws of inheritance.

The supernumeraries, which have a satellite DNA, occur in warm, dry environments, and are scarce or absent in humid, cooler localities.

[citation needed] B chromosomes have a tendency to accumulate in meiotic cell products resulting in an increase of B number over generations, thereby acting as selfish genetic elements.

For example, it has been discovered that the supernumerary chromosome of the pea pathogen Haematonectria haematococca carries genes that are important to the disease-causing capacity of the fungus.

This supernumerary DNA was found to code for a group of enzymes that metabolize toxins, known as phytoalexins, that are secreted by the plant's immune system.

[5] It is possible that these supernumerary elements originated in horizontal gene transfer events because sequence analysis often indicates that they have a different evolutionary history from essential chromosomal DNA.