Quadruplets are unusual enough, with about 500 sets born each year in the United States, according to the National Center for Health Statistics. But the daughters of mother Ornsee Khamsa and father Verek Muy of West Sacramento are genetically identical, originating from a single egg fertilized by a single sperm, doctors said. Such a phenomenon has been documented only a handful of times in recent history, according to Jennifer Cassidy, a Canadian who maintains a Web site devoted to multiple births. Cassidy said she is aware of only 26 sets of identical quads born since 1930, 17 ...