The Romney Campaign says we need to look at his Anglo-Saxon heritage. OK. Let's see which parts of Romney are Anlgo-Saxon and try to better understand their story and how they fit into the Romney family tree.
Mitt Romney finds his heritage linked to two other distinct lines, the Pratt family, which is Anglo-Saxon, and the Wilcken family which is German.
Here you can see a family tree for Mitt Romney. Below, I've written in the general heritage of the branches. I've also highlighted the polygamous marriages in pink to avoid any unnecessary confusion.
Beginning at the top left, Parley P. Pratt's people come from England and are Anglo-Saxon. Parley was the first to join the Mormon church. He had 12 wives and was murdered by the husband of the woman he married as his 12th. It's an interesting story, and you can read about it at the wikipedia link above. Helaman Pratt, his son had several wives, two of them sisters. This is where the Wilcken and Pratt family line join.
Charles Wilcken (top of family tree) was born Karl Heinrich Wilcken in Germany (not Karl Heinrich Marx). He served in the Prussian army, then ran a mill his father bought in Denmark, escaped service with the Danish military and planned to go to Argentina but had too much fun in London and only had enough money to make it to New York. From there he joined the US army but deserted to join the "Mormon Rebellion." He was the first in his family to become a Mormon and he had several wives by the time of his death. His daughter, Anna Wilcken married Helaman Pratt as part of his polygamous marriage. They gave birth to Anna Amelia Pratt, who married Gaskell Romney and lived in Mexico with Miles Romney, his father and his wives.
Miles Romney was born to Miles Romney who converted to Mormonism in England and then left for America. Gaskell and Anna were not in a polygamous relationship. They gave birth to George W. Romney, who married Lenore Lafount (her father was born in England, who's father was born in England, who's father was born in France), and they in turn gave birth to Mitt Romney.