"Extrachromosomal inheritance" is a term to be avoided. It is used sometimes as a synonym for cytoplasmic inheritance, which usually arises from the genetic information on mitochondrial or chloroplast genomes. Using the term chromosome in the broad sense of any DNA or RNA molecule capable of autonomous replication, the term extrachromosomal inheritance has a very limited content (see Maternal effects).
If the term chromosome is restricted to the complex structures in the nuclei of the eukaryotes, then extrachromosomal inheritance covers both cytoplasmic inheritance and the inheritance of plasmids that replicate autonomously in the nuclei of the cells. Because their distribution patterns differ from those of the major chromosomes, their inheritance diverges from Mendelian rules (see Mendelian Inheritance). Most nuclear plasmids are phenotypically silent and can only be detected by identification of their molecules. This is the case of the "two-micron circle," a nuclear plasmid of Saccharomyces cerevisiae . Although it does not confer any phenotype to the yeast, it has been useful in the development of vectors for genetic engineering.
In bacteria, extrachromosomal inheritance could designate the inheritance of autonomous plasmids separate from the main chromosome (see F Plasmid).
