ROSTOCK, Germany—Karin Vogel wakes up in this graffiti-pocked east German city and drives to the hospital where she is a therapist who counsels elderly people in chronic pain.

If a few thousand people would just disappear, Ms. Vogel would be leading a far more enchanting life. She would be the queen of England.

Everyone knows that should 85-year-old Queen Elizabeth II die, her son Charles, if living, would succeed her. Second in line is Charles's son Prince William, whose wedding to Kate Middleton Friday will be a global media event. William's little brother, Prince Harry, is No. 3.

Ms. Vogel, 38, holds a different distinction: By the account of some genealogists, she is the last person in line to the throne.

Ms. Vogel stands behind the many who, like her, are descendants of Sophia of Hanover, a relatively obscure German princess selected by the English Parliament in 1701 to inherit the crown. Sophia's genes have dictated the succession ever since.