A Y-DNA test traces a man's father's father's father's.... etc. The patrilineal line. Since I am a Johnson male, if another Johnson male takes a Y-DNA test, then we ultimately have the same Y-DNA ancestor (i.e., our patrilineal ancestors were brothers). Ancestry, 23andMe, etc., are autosomal DNA (atDNA) tests which show ancestry on both sides. Autosomal DNA is a mixture of parents. Y-DNA is via the father to the son only. So it could help to trace the Johnson line. FamilyTreeDNA is where I recommend men take Y-DNA tests.