How to calculate cM's in a Triangulated Group?

+12 votes

I have a triangulated group where I need to take the highest start and the lowest stop to add a source to a profile page, but I am stumped on how to change the cM's to suit the adjusted match.  Any ideas?

The groups is

Chr Start Stop cM
9 99,908,721 135,908,483 49.4
9 99,962,228 120,368,347 25.8
9 112,209,452 135,951,119 34.7

So the actual triangulated start and stop point is:

112,209,452 - 120,368,347  
How many centimorgans would that be?


in The Tree House by Rhonda Lucas G2G6 Mach 1 (12.0k points)
What a great tool!!! I'd love to add something like this to my website that has atDNA tools:  I'm going to reach out to them to see how long there tools are maintained. :)

2 Answers

+3 votes
Best answer
I have used the Rutgers Map Interpolator:
by Ann Turner G2G6 Mach 1 (13.0k points)
selected by Rhonda Lucas
Can you explain how you use the Rutgers Map Interpolator to figure cMs?

Pick the chromosome (9), click on the radio button for relevant physical positions, and enter the Mb in the query box, one per line. In the case above that would be



Then click on Get Map Positions.

No.  Chr Query_Posn Sex_Ave_cM Female_cM Male_cM
1   9  112209452   117.5399  147.0747 89.4962
2   9  120368347   128.5183  159.9379 98.4486

Find the difference between the two positions, using the sex averaged column. So it would be 128 - 117 = 11 cM. 





Great! Thank you.
Thanks Ann, never heard of that one, and it is so super easy to use :)
+1 vote
GEDmatch's triangulation utility can tell you the cMs for just the three people in question.  What are their GEDmatch IDs?
by Peter Roberts G2G6 Pilot (612k points)

Which tool in particular Peter?  I have Tier 1 and have tried them all.

Here are the kits:

  • A889678 - A535558
  • A889678 - M033616
  • A889678 - T137121

relationship is confirmed by a triangulated group consisting of [[Lucas-407|Rhonda Lucas]] GEDmatch A889678, [[Miller-34043|Margaret Sealey]] GEDmatch T137121, [[Add lineage|Maria Milo]] GEDmatch A535558, [[Brophy-85|Shelly Brophy]] GEDmatch M033616 sharing a 0.0 cM segment on chromosome 9 from 112,280,302 to 120,391,730. 


A889678, T137121, and M033616 all match each other on an 11.1 cM segment on chromosome 9.

To figure this out, you can use GEDmatch's free Multiple Kit Analysis and their One-to-one compare.   Multiple Kit Analysis shows that on chromosome 9 A889678 matches T137121 from 112209452 - 135951119 (34.685 cM), and A889678 matches M033616 from 99962228 - 120368347 (25.774 cM).  To discover the number of centimorgans for the overlap between T137121 and M033616 you can then use the One-to-one compare which shows a 11.1 cM shared between them.  In other words the shared segment from all three on chromosome 9 is 11.1 cM.

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