About a decade ago, Barbie began losing market share in the high-value fashion doll industry after a small-time competitor, MGA Entertainment Inc., stole trade secrets and the designer at Mattel Inc. who had come up with the idea for the wildly popular Bratz dolls, John Quinn, Mattel’s lawyer, told a federal jury on Tuesday.

“This is a case about how one competitor here stole and created a whole product line through theft,” said Quinn, a partner at Los Angeles-based Quinn, Emanuel, Urquhart & Sullivan, in opening statements before a packed courtroom in Santa Ana, Calif. “Rubber and plastic and fake hair doesn’t make little girls — or the parents of little girls — buy dolls. It all starts with the genius and inspiration of the doll design.”