sciencehabit writes: Although humans have about 20,000 genes, exactly what most of them do inside our body's cells is still murky. One way to learn more is to find people who lack a working copy of a particular gene and see how that affects their health. Such so-called knockouts are scarce in the general population. But a new study points to a more efficient way to find them: Search the DNA of people from a culture in which marrying a relative is common. The study has found a number of genes that we seemingly can do without, including those thought to prevent serious diseases. And one healthy mother completely lacked a gene called PRDM9 that is involved in shuffling chromosomes during the formation of eggs and sperm. Mice lacking the gene are sterile.