If two persons are biologically related, they share long stretches of DNA that they co-inherited from their recent common ancestor. These almost identically shared stretches of genomes are called IBD ("Identity by Descent") segments. Up to the sixth-degree relatives—such as second to third cousins would be, or a great great great great grandparent—the two relatives even share multiple IBD segments. Personal genomics companies such as 23andme or Ancestry detect those segments routinely in DNA of their customers, and use this signal to distinctively reveal biological relatives in their databases.
