Males with the THERRELL surname or any of the many variants with T__r__l_ are invited to participate. We prefer that you order the 25-marker Y-chromosome test kit, especially if you use the THERRELL spelling. The 12-marker test can be ordered and can be upgraded later without retesting, however the total cost to do this is greater ($99+$90 vs $169). The ideal person to be tested in a family group is a male in the oldest living generation. When you order a kit, please send a gedcom or text file to the project manager showing the test subject's paternal ancestry (with spouses) back to the most distant known THERRELL.