Will pay for Y-DNA-37 for descendant of first wife of John Roley d 1822 Westmoreland Co, PA

Question

John Roley had two wive named Margaret. A descendant of the second Margaret is asking for a descendant along a male line of the first Margaret to take a Y-DNA-37 test and will pay for it at FamilyTreeDNA.

Comments

Forgive me, I'm not understanding. What will YDNA do other than prove two male-line descendants are descended from John Roley? (Or a Rolley male period). Is there reason to believe John didn't father some of his sons?

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If direct paternal line descendants of John Roley have sufficiently matching Y haplotypes then it is supporting evidence that the known genealogy is accurate.

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Also, there is another local Roley that should be John's son, Robert, but his Y-DNA does not match the "known" one.