Extend your 37 marker yDNA test to ??

On Facebook and in other genetic genealogy forums the question is often asked if it would be useful to extend a yDNA test. The conventional wisdom seems to be that mismatches occur in a symmetrical manner. According to that scenario, two men who mismatch by zero or 1 on a 12 marker test are likely to have 1 or 2 mismatches over 25 markers. If the test were to be extended to 37 markers, 2 or 3 mismatches might be expected. If the test were extended to 67 markers, 3 or 4 mismatches might be expected. Finally, if the tests were upgraded to 111 markers, one might expect to find 5 to 7 mismatches. The underlying assumption is that mismatches occur in a predictable manner.

However, DNA is not inherited in such an orderly way. At least we have yet to discover laws of inheritance that guide us to what to expect. RANDOM seems to be the controlling factor. If two men have 3 mismatches over 111 markers, it is possible that all 3 of them will occur in the first 12 markers. It is also possible that all three will occur between markers 99 and 111. Neither of these distributions is likely, but they are both possible.   

I have a 6th cousin -- once removed with whom I mismatch on ySTR marker #1 and also on marker #18. I have no other mismatches with him on markers #19 through #111. This cousin is the fourth man listed below in my baker's dozen of my matches over 111 markers. The two cousins listed just above him, are 110/111 matches. One is a mismatch on the 79th marker and the other is a mismatch on marker 100. Both are one generation closer to me than the cousin with whom I am a 109/111 match. Another 6th cousin -- once removed is an 11/12, 24/25, 34/37, 63/67 and 105/111 match.  

Y-DNA  12	Y-DNA 25	Y-DNA   37	Y-DNA 67	Y-DNA 111
0	0	1	2	6
0	0	0	0	1
0	0	0	0	1
1	2	2	2	2
1	1	2	4	7
0	0	2	2	3
0	0	1	1	2
1	1	1	2	6
1	1	2	3	9
1	1	2	3	10
1	1	3	4	6
1	1	2	4	7
1	1	2	5	10

Random reigns! The results from the first 12, 25 or 37 STR markers are not indicators of what the next 30 or 74 STRs may tell us. You won't know unless you test. 

Of course you may find out all you need to know to answer your genealogical question(s) without testing 111 markers.