micronucleus
/ˌmʌɪkrə(ʊ)ˈnjuːklɪəs/Zoology and Biology
noun
Originally: the smaller of the two types of nucleus of ciliate protozoans, which is involved in sexual reproduction. Later also: any relatively or abnormally small nucleus.
Origin
Late 19th century; earliest use found in Journal of the Royal Microscopical Society. From micro- + nucleus, after French micronucléus.