Richard Dawkins makes regular film cameos.
In addition to the normal karyotype , wild populations of many animal , plant , and fungi species contain B chromosomes -LRB- also known as supernumerary , accessory , -LRB- conditionally - -RRB- dispensable , or lineage-specific chromosomes -RRB- .. karyotype. karyotype. species. species. chromosomes. chromosomes. By definition , these chromosomes are not essential for the life of a species , and are lacking in some -LRB- usually most -RRB- of the individuals .. species. species. chromosomes. chromosomes. Thus a population would consist of individuals with 0 , 1 , 2 , 3 -LRB- etc. -RRB- supernumeraries .. Most B chromosomes are mainly or entirely heterochromatic -LRB- and so would be largely non-coding -RRB- , but some , such as the B chromosomes of maize , contain sizeable euchromatic segments .. chromosomes. chromosomes. heterochromatic. heterochromatin. euchromatic. euchromatin. In general it seems unlikely that supernumeraries would persist in a species unless there was some positive adaptive advantage , which in a few cases has been identified .. species. species. For instance , the British grasshopper Myrmeleotettix maculatus has two structural types of B chromosomes  : metacentrics and submetacentrics .. chromosomes. chromosomes. grasshopper. grasshopper. Myrmeleotettix maculatus. Myrmeleotettix maculatus. The supernumeraries , which have a satellite DNA , occur in warm , dry environments , and are scarce or absent in humid , cooler localities .. satellite DNA. satellite DNA. B chromosomes are not to be confused with marker chromosomes or additional copies of normal chromosomes as they occur in Trisomies .. chromosomes. chromosomes. Trisomies. Trisomy