Editing Selfish genetic element (section)

=== B chromosomes ===
[[B chromosome]]s refer to chromosomes that are not required for the viability or fertility of the organism, but exist in addition to the normal (A) set.<ref>{{cite book | vauthors = Douglas RN, Birchler JA | veditors = Bhat T, Wani A | chapter = B Chromosomes | title = Chromosome Structure and Aberrations | publisher = Springer | location = New Delhi | date = 2017 | pages = 13–39 | doi = 10.1007/978-81-322-3673-3_2 | isbn = 978-81-322-3673-3 }}</ref> They persist in the population and accumulate because they have the ability to propagate their own transmission independently of the A chromosomes. They often vary in copy number between individuals of the same species.

B chromosomes were first detected over a century ago.{{When|date=December 2021}}<ref>{{cite journal | vauthors = Wilson E | title = The supernumerary chromosomes of Hemiptera. | journal = Science | date = 1907 | volume = 26 | pages = 870–871 }}</ref> Though typically smaller than normal chromosomes, their gene poor, heterochromatin-rich structure made them visible to early cytogenetic techniques. B chromosomes have been thoroughly studied and are estimated to occur in 15% of all eukaryotic species.<ref>{{cite journal | vauthors = Beukeboom LW | title = Bewildering Bs: an impression of the first B-Chromosome Conference. | journal = Heredity | year = 1994 | volume = 73 | issue = 3 | pages = 328–336 | doi = 10.1038/hdy.1994.140 | doi-access = free }}</ref> In general, they appear to be particularly common among eudicot plants, rare in mammals, and absent in birds. In 1945, they were the subject of Gunnar Östergren's classic paper "Parasitic nature of extra fragment chromosomes", where he argues that the variation in abundance of B chromosomes between and within species is because of the parasitic properties of the Bs.<ref name=":6" /> This was the first time genetic material was referred to as "parasitic" or "selfish". B chromosome number correlates positively with genome size<ref name=":12">{{cite journal | vauthors = Trivers R, Burt A, Palestis BG | title = B chromosomes and genome size in flowering plants | journal = Genome | volume = 47 | issue = 1 | pages = 1–8 | date = February 2004 | pmid = 15060596 | doi = 10.1139/g03-088 }}</ref> and has also been linked to a decrease in egg production in the grasshopper ''Eyprepocnemis plorans''.<ref>{{cite journal | vauthors = Zurita S, Cabrero J, López-León MD, Camacho JP | title = Polymorphism regeneration for a neutralized selfish B chromosome | journal = Evolution; International Journal of Organic Evolution | volume = 52 | issue = 1 | pages = 274–277 | date = February 1998 | pmid = 28568137 | doi = 10.1111/j.1558-5646.1998.tb05163.x | s2cid = 2588754 }}.</ref>

[[File:Cell with transmission patterns.png|thumb|Genetic conflicts often arise because not all genes are inherited in the same way. Examples include cytoplasmic male sterility (see [[#Selfish mitochondria|Selfish mitochondria]]). While mitochondrial and chloroplast genes are generally maternally inherited, B chromosomes can be preferentially transmitted through both males and females.]]