单选题 .  We all know that DNA has the ability to identify individuals but, because it is inherited, there are also regions of the DNA strand which can relate an individual to his or her family (immediate and extended), tribal group and even an entire population. Molecular Genealogy (宗谱学) can use this unique identification provided by the genetic markers to link people together into family trees. Pedigrees (家谱) based on such genetic markers can mean a breakthrough for family trees where information is incomplete or missing due to adoption, illegitimacy or lack of records. There are many communities and populations which have lost precious records due to tragic events such as the fire in the Irish courts during Civil War in 1921 or American slaves for whom many records were never kept in the first place.
    The main objective of the Molecular Genealogy Research Group is to build a database containing over 100,000 DNA samples from individuals all over the world. These individuals will have provided a pedigree chart of at least four generations and a small blood sample. Once the database has enough samples to represent the world genetic make-up, it will eventually help in solving many issues regarding genealogies that could not be done by relying only on traditional written records. Theoretically, any individual will someday be able to trace his or her family origins through this database.
    In the meantime, as the database is being created, molecular genealogy can already verify possible or suspected relationships between individuals. "For example, if two men sharing the same last name believe that they are related, but no written record proves this relationship, we can verify this possibility by collecting a sample of DNA from both and looking for common markers (in this case we can look primarily at the Y chromosome (染色体)," explains Ugo A. Perego, a member of the BYU Molecular Genealogy research team.1.  People in a large area may possess the same DNA thread because ______.