GUIDELINES FOR COUNTING MISMATCHES OF DNA RESULTS

When calculating the differences (sometimes called mismatches or mutations) between two sets of DNA test results, the usual procedure is to total the number of markers that are different. However, there are several special situations that need to be taken into account.

DIFFERENCES OF MORE THAN ONE ON A MARKER

When there is a difference of more than one in the numerical values for a marker, this is usually treated as a mismatch of one. The assumption is that a mutation created this larger diffence in a single step rather than in multiple steps. However, that may not always be correct. Sometimes a difference of two might have been caused by two separate mutation, and there are even reverse mutations that can occur. However, since it is usually difficult or impossible to determine exactly how mutations occurred, the counting of a difference greater than one as a mismatch of one is reasonable.

MARKERS 389i AND 389ii

Marker 12 (389ii) includes marker 10 (389i). Therefore, if both markers change by one in the same direction, that is really only a mismatch of one. An example of this is sample 1353 in the red group of the master DNA results table. Although it looks as if it has 3 mutations, it really only has 2.

MARKERS 464a-d

(Most of the explanation for this set of markers is from the Family Tree DNA website.) Markers 464a-d are copies found at different locations on the Y chromosome. The reporting of genetic distance for Markers 464a-d has changed from its original concept. Previously, when comparing the results for these Markers, each result that did not match was considered a mismatch. With new guidelines from the scientific board for Family Tree DNA, the genetic distance calculation for Markers 464a-d has been revised. Results are always reported from low to high, when reading from left to right. When a mismatch occurs, it must be taken into consideration whether the number of apparent mismatches are a result of the order of presentation of the Markers. The order of the results for these Markers may make it appear as if there are more mismatches than are actually present. Here are two examples:

Results Genetic Distance

13 15 16 16

13 15 15 16 1



16 16 17 18

15 16 16 18 1

In the first example, the mutation did not cause the results to be reordered, so it is very clear that there is one mismatch. In the second example, if a 2 point mutation occured (17-2 = 15) the loss of 2 repeats caused the results to be reordered. On the surface, it looks like there are two mismatches, but this illusion is caused by the results being reordered, and there is only 1 mismatch, the 17 becoming the 15. Remember, DYS 464a-d is a highly polymorphic Marker, in fact it appears to be the fastest moving marker in the entire test. (Polymorphic means rapidly changing!)

Last Revised 3 April 2003