The GEDmatch.com Chromosome Segment Matching report has the following legend:

Chromosome browser legend

One of my reports has the following triangulation match:

enter image description here

The purple segment (< 5 cm) is 102912701 - 111046063 (4.189 cM) and the green segment (10-20 cM) is 149430788 - 154663050 (11.939 cM).

Since the green segment is almost 3 times as many cM, why does the visual show the purple segment being almost twice as long? Is the match somehow spread out with bits of non-matching portions mixed in, thus getting elongated?

Also, what does the F2 type mean (not show in image)? I'm getting F2 on each matching person's table entry above the graphic such as the shown one above.

Does such a matching segment mean the two kits are identical in the regions shown?

1 Answer 1


The colored segments are not drawn in terms of cM, but in million base pairs (Mbp), which are the number shown on the bottom of your graphic. The numbers at the bottom are very approximate in position.

So your purple segment is drawn between 102.9 and 111.0 Mbp and your green segment is drawn from 149.4 to 154.6 Mbp. So the purple segment is 8.1 Mbp long and the green segment is 5.2 Mbp long and that is why the purple segment is longer than your green segment.

It just so happens that the green segment has more cM than the purple segment. That is entirely possible. cM or Centimorgan, is not a measure of length, but is an approximate measure of how often the segment is expected to crossover in one generation. 11.9 cM means that there is approximately an 11.9% chance that this segment will crossover in the next generation. A crossover is where the father's chromosome half switches with the mother's and a crossover will change the part of the chromosome that will match in the next generation.

Some parts of the chromosome subdivide more often than others, and it so happens that your green segment subdivides 3 times as often as your purple segment. That is important, but it's not what's graphed.

The F2 your are talking about (as in the below image):

enter image description here

is the company and version of the file that the DNA information comes from:

enter image description here

It has nothing to do with whether the kits are identical or not.

  • I'm asking if the kits are identical in the colored matching segments?
    – WilliamKF
    Commented Sep 28, 2016 at 17:58
  • Each row in the display shows the segments of one person. That is represented by the "1" at the left and the right of your graphic and by the "Match ID" in my graphic. So yes, the segments in that row, the purple and green segments, both use the same kit, the kit of person with ID 1.
    – lkessler
    Commented Sep 28, 2016 at 19:26
  • The legend offers >100 cM, so how could the change of crossing over be greater than 100%?
    – WilliamKF
    Commented Sep 30, 2016 at 1:05
  • 1
    To be more precise, cM is actually the expected number of crossovers on the segment in one generation multiplied by 100. So for example, a 120 cM segment might get 0, 1, 2, or even 3 or more crossovers, but should average to about 1.2 crossovers. The DNA totals about 3800 cM, and averages about 38 crossovers per generation over all the chromosomes. For small values of cM, e.g. 11.9 cM, the chance 11.9% of a crossover on that segment is approximately equal to 0.119 the expected number of crossovers.
    – lkessler
    Commented Sep 30, 2016 at 13:07

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