LESSON 12: Chromosome Mapping

If you have made it this far you are no longer a beginner! Chromosome Mapping is for the serious genetic genealogist. It is time consuming, but also very rewarding. It is the only way to truly know that a segment of DNA comes from an identified ancestor or ancestor pair. It is the process genetic genealogists use to track matches and then confirm our matches' relationship to us. You may need to collect quite a bit of data before using the mapping tools.

In our previous lessons you have already seen chromosome maps. Chromosome maps are simply graphic representations of shared DNA on an illustration of our chromosomes. The data used to illustrate or paint the maps is available by download from these sources depending on where you have tested and uploaded. Note: for those who have only tested at ANCESTRY you will need to upload to FTDNA or GEDMATCH to be able to map matching segments.

FTDNAFamily Finder --> Chromosome Browser -->Download all Matches to Excel 

23andMeto Download all matches at once go to Family and Friends --> DNA Relatives then find download at bottom of page. Or to download select matches go to About Me --> Ancestry Overview -->Ancestry Tools --> Ancestry Inheritance: Advanced --> View in Table or Download Table

GEDMATCH: You must register and upload your RAW DNA file first. This is currently the only way to see ANCESTRY matching segments if they have not been uploaded to FTDNA. This is a free site. Please consider donating to keep it going. Once registered log in and select Analyze Your Data--> One-to-One Comparison --> enter kit numbers  and select Do not Show Graphic Representation Bar OR Analyze Your Data--> One-to-Many Matches--> tick those you want to compare --> chr. browse

DNAGedcom.com: an easier way to download either/both FTDNA or 23andMe. You must register and then follow the instructions. Note: This site was developed to assist adoptees. Please consider donating if you use this site as it's for a good cause.

Use a spreadsheet to keep your data organized. You may use the data exactly as downloaded or add additional columns for your own use. They might include email, notes, paternal or maternal, etc. Once data is copied into the master file I like to sort it by chromosome and segment start in order to look for overlaps. If the data comes from 23andMe I can check to see if my matches' segments match each other which allows me to phase sides, especially if they happen to match one of my known cousins. Once I know which side a match is on (maternal or paternal) I personally change the type to red or blue. I also shade groups of matches. There are many ways to do this and it can be personalized however you want. (See more examples in resources below.) This is a part of my spreadsheet.

                                           Match Pair   chromosome   start      stop     CM      SNPs    Notes

You will see that some of the red numbers (on this screen shot) have more detail. These are from FTDNA whereas those from 23andMe are rounded off. Triangulation is a process of confirming that a segment's origin is indeed where we think it is. If we have 3 individuals with an overlapping segment and they all match each other AND have one or more ancestors in common yet have different lines of descent then we assume the segment was inherited from the common ancestor(s). We have triangulated the ancestor(s) match. Let us say Rebecca Smith, Deborah Jones and Sarah White all match on the same segment. Their lines of descent are shown in the chart below from their common ancestors Peter Jones and Deborah Haley.

Peter Jones m. Deborah Haley

3rd great-grandparents

Sarah Jones (m. John Smith)   


Peter Jones Jr.


Robert Jones



2nd great-grandparents

John Smith Jr


Gordon Jones


Robert Jones Jr.


1st cousins

1st great-grandparents

Frank Smith


Ezra Jones


Elizabeth Jones m. Doug Tate


2nd cousins


John Smith


Peter Jones


Sarah Tate m Ed White


3rd cousins


Rebecca Smith

Deborah Jones

Sarah White

4th cousins


Since they all share the same set of 3rd great grandparents and the same segment of DNA we can assume the segment came from this couple. For each they can ascribe it to their 2nd great grandparents since it came down to each through them.

Once you have this data organized you can use it to "paint" your own "Chromosome Map." Below you can see mine using Kitty Munson Cooper's "Chromosome Mapping Tool." Kitty built this after my plea for a programmer to do just that. This allows you to paint the contributions of your 16 great-great grandparents or up to any 20 ancestors of your choosing. Please consider making a contribution to Kitty if you enjoy her tool. She has subsequently developed a "Segment Mapping Tool" for those wanting to paint multiple segments with self-defined labels. More information is available below. The latter is extremely versatile. Both are destined to be very popular with genetic genealogists.

My Chromosome Map
from the
Chromosome Mapping Tool
tool courtesy Kitty Munson Cooper

Chromosome Segment Mapping Tool

image & tool courtesy Kitty Munson Cooper

Find New ancestors with DNA. by Shari Simonds

Chromosome Mapping by Tim Jantzen