What if the DNA sequence of an individual explains only part of the story of his or her inherited diseases and traits, and we need to know the DNA sequences of parents and perhaps even grandparents to understand what is truly going on? Before the Human Genome Project and the era of widespread DNA sequencing, those questions would have seemed ridiculous to researchers convinced they knew better. But modern genomics has run into a Mendelian wall. Large-scale genomic studies over the past five years or so have mainly failed to turn up common genes that play a major role in complex human maladies.