It's fascinating that the imprint of father to son continues on so clearly in the y chromosome so that test is pretty determinative.
One site, familytreedna gives you a visual presentation of how your chromosomes match up to other relatives. Below are three visualizations of the chromosome match between 1) both parents and a child, 2) both grandparents and a grandchild, and 3) a second cousin.
|both parents chromosome match with a child (the gray areas not compared)|
|both grandparents (one one side) match with grandchild (gray areas not compared)|
|2nd cousin chromosome match (gray areas not compared)|