Chromosome mapping is a technique used in autosomal DNA testing which allows the testee to determine which segments of DNA came from which ancestor. In order to map DNA segments on specific chromosomes it is necessary to test a number of close family relatives. Ideally one should test both parents, one of their children, and a number of first to third cousins on both the maternal and paternal sides of the family.
Not everyone has close relatives available for testing or has the funds to pay for such testing. Indeed, even if you did test all of your first and second cousins you might not be able to map your entire genome. In any case, the more first and second cousins you test, the higher the percentage of your genome that you can map, at least back to which parent or grandparent contributed any particular DNA segment.
Mapping specific DNA segments to early ancestors is more challenging, particularly in endogamous (inter-married) populations. In such situations it is helpful to use a complementary technique known as triangulation. This involves finding other distant cousins who share the same segment and who share the same common ancestor. The caveat is that without chromosome mapping you cannot be sure that the documented common ancestor is the one who provided the shared DNA segment. The more matches who share both a specific DNA segment and a specific ancestor with you, the higher the probability that the DNA segment came from the common ancestor.
One should be particularly cautious about mapping segments to people who are related to you at no closer than about the 6th cousin level without additional corroborating evidence, such as two or more people who share the same segment with you (or your parents) and who also share the same common ancestor.
You need to keep in mind that the shared segments as reported by the DNA testing companies such as 23andMe and Family Tree DNA are simply half-identical regions. Half-identical regions (HIRs) may be either identical by state (IBS) (false matches) or identical by descent (IBD) (true matches). Using phased data for the comparisons makes it much easier to determine which half-identical regions are Identical by State and which are identical by descent.
In order to map DNA segments it is necessary to maintain a list of your matches, preferably in a spreadsheet. The matches spreadsheet should include the names and contact details of your matches, details of the matching segments and number of shared SNPs, the name of the most recent common ancestor and the known relationship.
ISOGG member Tim Janzen has been one of the leading researchers using the technique of chromosome mapping. See his post on the Genealogy DNA Rootsweb mailing list dated 7th June 2011 which provides information on the methodology used. The post can be found at http://archiver.rootsweb.ancestry.com/th/read/AUTOSOMAL-DNA/2011-06/1307493819.
Tim Janzen and Emily Aulicino have prepared a 16-page tutorial on the "Basics of chromosome mapping" which describes how to create a list of all one's matches and their accompanying HIR (half-identical regions) data. The file can be downloaded here.
Kitty Cooper has provided a number of tools for mapping segments and chromosome mapping which are listed below.
Blog posts and articles
- Chromosome mapping by Kelly Wheaton. Lesson 12 in the series "Beginner's Guide to Genetic Genealogy", Wheaton Surname Resources website, 2013.
- Identifying DNA from great grandparents using second cousin comparisons by CeCe Moore, Your Genetic Genealogist blog, 25 September 2011.
- A second cousin adds to my chromosome map and answers a nagging genealogical question by CeCe Moore, Your Genetic Genealogist blog, 5 July 2013.
Kitty Cooper's chromosome and mapping tools
- Kitty Cooper's Excel file for tracking matching DNA segments
- Kitty Cooper's segment mapping tool
- Kitty Cooper's one-chromosome mapper A tool which allows the user to look in detail at just one chromosome. A chart can be displayed for up to 40 people who have segment matches with you on a single chromosome.
Blog posts and reviews relating to Kitty Cooper's tools
- Making a spreadsheet of autosomal DNA matches by Kitty Cooper, Kitty Cooper's blog, 15 November 2012.
- Building a color-coded chromosome mapping tool by Kitty Cooper, 22 June 2013.
- First version of chromosome mapping tool by Kitty Cooper, 31 July 2013
- Chromosome mapping tool released by Kitty Cooper, 3 August 2013
- Chromosome Mapper by Kitty Cooper – First Look by Rebekah Canada, Haplogroup blog, 4 August 2013.
- Chromosome mapping by Jody Lutter, Family History Research by Jody, 5 August 2013.
- Results in from my double 3rd cousin! by Kitty Cooper, 13 August 2013