Petty found Antwan Goodley and Levi Norwood on scoring strikes after halftime, breaking a 3-3 tie, and the Bears (11-1, 8-1 Big 12) took advantage of Oklahoma State's loss to Oklahoma earlier Saturday to grab their first-ever conference crown.
The regular season-ending victory in frigid conditions also assured Baylor a berth in the Fiesta Bowl.
Petty completed 21-of-37 for 287 yards and Glasco Martin racked up 102 rushing yards with a TD on 22 carries.
Goodley also made eight receptions for 114 yards.
The Longhorns (8-4, 7-2) will settle for a non-BCS bowl game after Case McCoy was held to just 54 passing yards on 12-of-34 efficiency.
Malcolm Brown led the Texas ground game with 131 yards on 25 carries and made a touchdown reception.
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