Here’s an example: James Zavacky and Peter Zavatsky want to find out whether they’re related, so each takes a 25-marker Y-DNA test. In comparing their results, they discover that 24 of those 25 markers are identical, with a mutation at marker 449:
That 24/25 match indicates that the men do indeed share an ancestor.
But what about the mismatching marker? Each value here shows how many times a genetic pattern repeats itself on that particular marker. James Zavacky and Peter Zavatsky don’t have the same number of “repeats” on marker 449 — but their values are close. That mutation in the Y chromosome signals that the men aren’t as closely related as men who match perfectly.