Simplify the following text for a Grade 6 native English-speaking person.
Original: Note , however , Yuan Dynasty is traditionally often extended to cover the Mongol Empire before Kublai Khan 's formal establishment of the Yuan in 1271 , partly because Kublai had his grandfather Genghis Khan placed on the official record as the founder of the dynasty or Taizu .
Simplified: His grandson Kublai Khan added more parts of China to his kingdom . He founded the Yuan dynasty in 1271 .