Isaac is identified by descendants of John Mertz of Auglaize County as his oldest son. A message board post says he was mentioned in his father’s will. His first wife was Eliza and then he married second Jane Allen in 1867.

Isaac is designated P5a1a.

As stated in my notes for Isaac's father John, some of the information about John's descendants comes from Mary Lou Clegg. She specifically notes the fact that Isaac is listed in the 1850 Census in Blythe Twp, Schuykill Twp, PA and indeed I have checked and Isaac Mertz is there with people whose names and ages basically match Isaac's 1860 listing in Auglaize County.

It must be the same Isaac, but does this imply the family went to Middleburg, then Schuylkill then Ohio? Or did Isaac do that alone among his family and then caught up them with later?

Isaac married, apparently a second time, in 1867, the widow Jane Allen and in 1870 he lived with her and her children. Then in 1880, he lived with his daughter Susan and her family.

Isaac is not on find-a-grave.