In 2015, The Independent newspaper reported the case of a man who had ‘failed’ a paternity test in the US because the genetic material in his saliva was shown to be different from that in his sperm. This was apparently the first reported instance of a paternity test being ‘fooled’ by a ‘human chimera’. 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 man’s deceased twin, who had never been born. Cases of chimeras potentially present a challenge to legal systems, given their frequent emphasis on genetics in determining parenthood. This talk will consider what the phenomenon of the chimera might tell us about our understanding of parenthood and the differences between biological motherhood and fatherhood respectively. Dr Sloan will advocate the recognition of the chimeric person as the ‘true’ legal father and suggests two methods through which this might be accomplished, pointing out the potentially significant and even damaging implications of one of them for English law’s general approach to parenthood. The talk also explores the likely practical response of English law to the situation of a potential chimera.