Gregor Mendel discovered fundamental rules of genetics by raising pea plants. He realized that hidden factors — we now know them to be genes — were passed down from parents to offspring.

It wasn’t until the early 1900s, long after Mendel’s death, that doctors discovered that humans weren’t so very different. Some diseases, it turns out, are inherited — they’re Mendelian.

Today, scientists have identified over 7,000 Mendelian diseases, and many are discovered with screenings of children and adults. But a new study suggests that many disorders go undetected.

With a database of electronic health records and DNA samples, a team of scientists has found that 3.7 percent of patients in a hospital system carried a genetic variant linked to a disease. It’s possible that as many as 4.5 percent of cases of apparently nongenetic diseases, from infertility to kidney failure, are the result of such mutations.