>Hello. I would like to know what test to do for two families sharing
>the same surname in a small southern town, but supposedly not
related?

A male from each family (more than one per family is better)would need to get the Y-chromosome test. But before doing that, have you done any research on these families?

>Also I'm trying to find out how some Henry's are cousins to me,but
>nobody knows how?Plus another scenario is a lady stated to me that
>our family lines were related without offering any proof.What should
>I do there?

Research, research, research. That was one of my reasons for starting genealogy “How am I kin to Cousin Noble? Cousin Esther? Those other folks with the same surname?” Perhaps back a few generations someone surnamed HENRY married someone in your family. That happened with my two aunts by marriage. They did not know how they were kin to the HEARNs. Well, it was from their grandparents' marriage. Their grandmother was a HEARN, but they didn't know that.

>Also my paternal grandfather had a different mother than
>hisiblings,so is it logical to trace both his natural and step-mom
>lines?

That is a choice you will have to make. Your grandfather’s mother is your ancestor, while his step-mother is the ancestor of your cousins. You may want to research both lines. My paternal grandparents each had children by three spouses; the 3rd marriage for both produced my father who is the youngest of the whole lot. But I am researching all of the spouses since their children are my aunts and uncles.

>I know very little about my grandfather birth mom. Both of my parents
>are still living so if I'm clear on this DNA then I could use my
>father DNA to help me follow 16 great great grandparents lines that
>his DNA would be reflective in 8 of those 16?

Unfortunately, no. The DNA testing of your parents allows you to trace the lineage of only 2 people in each generation of your father’s ancestors and only 1 person in each generation of your mother’s ancestors.

Testing your father: (1) the Y-chromosome test follows only his father’s father’s father’s father's etc., etc. line and (2) mtDNA follows only his mother’s mother’s mother’s mother’s etc., etc. line. That will give you the lineage of two great-parents, one male and one female, but not a couple.

Testing your mother: her mtDNA follows only her mother’s mother’s mother’s mother’s etc., etc. line, and can give you the lineage of only one greatgrandmother. In order to trace her father's line, you'd need to test a brother of hers, or the son of her brother, assuming you mother and her brother have the same father.

Here's where you need to look at cousins. Depending on who is living, you may want to test your grandfather's sister, for example, or her daughter. For example, my parents are deceased, so I am testing my father's sister's daughter's daughter in order to get the lineage of my father's mother.

toot