Outside or separated from, a chromosome.
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ex·tra·chro·mo·som·al -.krō-mə-'sō-məl, -'zō- adj situated or controlled by factors outside the chromosome <\extrachromosomal inheritance> <\extrachromosomal DNA>
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ex·tra·chro·mo·so·mal (eks″trə-kro″mo-soґməl) outside or not involving the chromosome, as in mitochondrial inheritance, which involves only mitochondrial DNA.Medical dictionary. 2011.