jats:titleAbstract</jats:title>jats:pIn apparently the first reported instance of a paternity test being ‘fooled’ by a ‘human chimera’, a man ‘failed’ a paternity test because the genetic material in his saliva was shown to be different from that in his sperm. Such a chimera has extra genes, here absorbed from a twin lost in early pregnancy. The result was that the 'true' genetic father of the man's son was the twin, who had never been born. Chimeras present a challenge to legal systems, given the frequent emphasis on genetics in determining parenthood. This paper considers the phenomenon's potential implications for English law's understanding of parenthood. It advocates the recognition of the chimeric person as the ‘true’ legal father and suggests two methods through which this might be accomplished, pointing out their broader consequences. The paper also explores English law's likely practical response to the situation of a potential chimera.</jats:p>