Of over 7000 patients referred to a diagnostic laboratory, 28% had diagnoses based on DNA sequencing, 5% of whom had two or more diagnoses. Their phenotypes could be better understood by considering whether the implicated genes affect independent biologic processes or organ systems. Medical genetics focuses on the relationship between observed phenotypes and their underlying genotypes, modes of transmission, and risks of recurrence. Expected patterns of mendelian inheritance are often used to confirm the identification of disease genes, and deviations from mendelian expectations have led to the discovery of more complicated genetic underpinnings of disease (Fig. S1 in the Supplementary Appendix, available with the full text of this article at NEJM.org). 1 – 8 Multiple (or dual) molecular diagnoses involve more than one clinical diagnosis and more than one genetic locus (Figure 1), each segregating independently. Diagnostic whole-exome sequencing affords opportunities for providing insights into relationships . . .