Utility of the X chromosome pattern of inheritance: the identification of close relatives through direct-to-consumer (DTC) genetic testing. K. Johnston Retired from HealthCare Partners Medical Group, 3565 Del Amo Blvd. Torrance California 90503.

   Members of the public who are looking for their birth parents can now order autosomal tests directly from ancestry companies to aid in their search for cousins who carry identical by descent (IBD) DNA segments. Females who are looking for their birth fathers, and both males and females who are looking for their birth mothers can utilize the matching X chromosome IBD segments they receive simultaneously from these companies. Extensive pedigrees compiled by close cousins who are related by way of the X chromosome provide useful links. The inheritance pattern of the X chromosome is unique. When studying pedigrees, all lineages that show two males in a row (father to son) can be eliminated from the family tree. X chromosome tracking aids in the identification of the most recent common ancestor(s) between two closely related DNA cousins. In addition, extensive online genealogical records submitted by the public, especially those that include living relatives, facilitate in the identification of collateral lines. In some cases, paternity testing is no longer needed to confirm a lineage when close relatives (e.g. half-siblings) are contacted, then are willing to be tested and matched through one of the DTC ancestry companies or online third party tools. Unsuspecting birth fathers, sperm donors and parents who have put up a child for adoption can now be traced through online pedigrees provided by closely related genealogists with the help of public records and familial DNA testing. An example involving two recently matched second cousins will demonstrate the steps involved.