TOKYO - Japan has opened the possibility of welcoming into the imperial family patrilineal male descendants from former imperial branches through adoption. The former imperial branches that will be eligible for adoption trace a common ancestor with the current imperial family back about 600 years to Prince Sadafusa of the Fushimi branch. Although it may be seen natural to make them part of the imperial family, they must remain commoners to avoid having matrilineal imperial family members. Female imperial members will be registered in the imperial genealogy, while spouses and children will be recorded in Japan's family register system. Having the throne passed down through the male line is an imperial tradition, and the male line guarantees the emperor's legitimacy.