## Why are the lengths GEDmatch.com Chromosome Segment Matching segments not proportional to their cM?

5

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

One of my reports has the following triangulation match:

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?

6

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.