Abstract
The variability of disease expression often complicates clinical classification. Since the 1960s, medical geneticists have sought to address this problem by associating diseases with discrete locations in the human genome. While this nosological approach was quite successful in the 1980s, unanticipated complications arose. In 1987, two historically distinct disorders, Prader-Willi and Angelman syndromes, were unexpectedly associated with the same genomic "address." Does genomic overlap imply nosological sameness? This paper explores the clinical and biological implications of this finding, and argues that the process by which it was resolved represented new modes of thinking and practice in late-twentieth-century biomedicine. In the decades before the completion of the Human Genome Project, the genome was understood to be, at once, a standardized scientific object and an observable part of the human anatomy. Depicted and analyzed at the level of chromosomes, the genome became an important conceptual space and experimental system for late-twentieth-century biomedicine, producing novel research questions that drew the attention of both clinically oriented physicians and basic laboratory geneticists.