Carolyn Abraham explores the stunning power and ethical pitfalls of using genetic tests to answer questions of genealogy--by cracking the genome of her own family.      Recently, tens of thousands of people have been drawn to mail-order DNA tests to learn about their family roots. Abraham investigates whether this burgeoning new science can help solve 2 mysteries that have haunted her multi-racial family for more than a century. Both hinge on her enigmatic great-grandfathers--a hero who died young and a scoundrel who disappeared. Can the DNA they left behind reveal their stories from beyond the grave?