I am indebted to William Alexander Roper, Jr. (mailto:firstname.lastname@example.org) for the following thoughful analysis of Y-chromosome markers probabilities:
After 10 generations, the MOST LIKELY marker comparison outcome for 12 markers is
So the most likely outcome after 10 generations is that all of the markers match, but the prospect of a 1-marker difference is actually pretty high. And the chance of a 2 marker deviation is certainly not inconceivable. Put another way, if you randomly selected 20 test subject all known to share a common ancestor 10 generations back, you would expect to have one of these 20 come up with a 2-marker deviation.
After 15 generations, the most likely marker comparison for 12 markers is:
By the 22nd Generation, there is a crossover, and it is more likely that there will be a 1-marker difference (37%) than no difference (35%). Also, the prospect that 2 markers differ by 1 has grown to 19% and the chance of a 3-marker difference is more than remote at 7%. The 4-marker difference (1.8%) and 5-marker difference (0.4%) are still rather unlikely with the 12-Marker Test.
The expected distribution for the 25-marker test is even more interesting. After only 10 generations, a 1-marker difference (36.82%) is already negligibly more likely than no difference (36.75%). A 2-marker difference (18%) would not be unusual, and a 3-marker difference (6%) would be expected in a sample of twenty random test subjects known to be related with an MRCA ten generations back!
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