There are several different categories of genealogy DNA tests.  The most common type is "autosomnal DNA."  This is the test sold by ancestry.com and 23 and Me and sold under the name Family Finder by FamilyTreeDNA.  It helps you locate cousins and learn more about all your ancestors.

Y-DNA is a different type of test.  Y-DNA passes from father to son to son to son and mutates very slowly.  A Y-DNA test tells someone who is male a great deal about his direct male line ancestors and nothing about any other line of ancestors.  As far as I can tell, in the U.S., Y-DNA testing is done only by FamilyTreeDNA. 

You can have different numbers of Y-DNA markers tested.  FamilyTreeDNA currently offers tests for 37 markers and 111 markers.  In the past, they offered tests for 12 markers, 25 markers, and 67 markers.  If two men with the same surname have most of their Y-DNA markers match, the odds are strong that they share a common direct male line ancestor in the not-too-distant past.  

I have had 111 markers tested.  A direct male line descendant of my great, great grandfather, Samuel McCrea, had a 67 marker test, so 67 of our markers can be compared.  Of those 67 markers, 65 match.  That confirms that we share a common direct male line ancestor in the not-too-distant past.  Since 2 of our markers don't match, he and I are said to be a "genetic distance of 2 on a 67 marker test."  There is one man with a direct male line ancestor whose surname is a variation of McCrea who is a genetic distance of 3 from me on a 67 marker test, two who are a genetic distance of 4 on a 67 marker test and two who are a generation of 5 on a 67 marker test.  Three of those men completed 111 marker tests.  One is a genetic distance of 8, one is a genetic distance of 9, and one is a genetic distance of 10 from me on a 111 marker test.  I have communicated with these men and we agree that we share a direct male line ancestor, but that was probably before our McCrea ancestors came to America

I opted to have my results appear onThe MacRae Clan DNA Project web page which shows Y-DNA results for men with last names that are variations of MaRae.  The results can be found by linking here.  They are grouped into kits that show a very likely common direct male line ancestor.  My results are shown as Kit #93990 with earliest documented male ancestor Samuel McCrea 1822-1884, Fair Hill, Cecil Co., MD.  I am in group R1b - McCrea #2.  The group heading has a bright yellow background.  Each of the columns in the chart show the results for one Y-DNA marker.  If the numbers are the same for two test kits, that marker is a match.

 

 

 

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