Is there a simple conversion for megabase to cM?

Question

When doing triangulation with multiple matches, the cM for each match might be known but the common match will be the highest of the start locations and the lowest of the stop locations and the cM for that range is unknown.  Is there an acceptable interpolation method or does one need to look at the frequency maps.  Ideally, there is an on-line calculator!

Answer 1

The way I do this on GEDmatch.com is change who is the anchor kit for multi-kit analysis so that the minimum spanning cM is one of the reported numbers.

For example, if Alice, Bob, and Charlie were being compared and Alice was the root and she showed a 16 cM match with Bob on Chromosome 2 that overlaps a 15 cM match for Charlie, but Charlie starts before Bob and Bob extends past Charlie, then I would rerun the report and put either Bob or Charlie as the first listed kit so that I'd get the cM in common between Bob and Charlie reported.

Comments

Yeah, that makes sense.  [Hanging my head low wondering why I didn't think of that!]

---

I faced this problem myself for a while, entering rough estimates by hand, before I thought of it either.  :-)

---

So obvious ... and so simple.

Answer 2

You have to have a map. It is not a fixed conversion rate, different areas have different recombination chances.  Calculating cM is one of the Hard Things that the scientists did for us to make genetic genealogy possible for the layperson.
It would be nice if there is an online calculator,  but I am not aware of one.

Comments

Right, one typically needs to use maps and it gets very burdensome I would imagine so unless there is a magic bullet such as the non-existent calculator AFAIK, then the question is how do we resolve that in WikiTree?  Is there an interpolation of the known cM values that is in the ball-park and acceptable for declaring a match?

Answer 3

You can use the Rutgers Map Interpolator. FTDNA and GEDmatch use Build 36; 23andMe and AncestryDNA use Build 37.

You pick a chromosome, then put the two (or more) Mb numbers on separate lines. Use the sex-averaged values to get the difference between the two points.

http://compgen.rutgers.edu/mapinterpolator

This calculator has been off-line for a while. It has a new work I'll have to look up, Kosambi cM.

Comments

It isn't offline, but it has a slightly different URL now.

http://compgen.rutgers.edu/map_interpolator.shtml

Answer 4

This is the new Rutgers calculator (which I think uses the Kosambi functions)

http://compgen.rutgers.edu/map_interpolator.shtml