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Chromosome mapping

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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.

Caution should be exercised when attempting to map smaller segments, and particularly segments under 10 cMs. You need to keep in mind that the shared segments as reported by the DNA testing companies are simply half-identical regions. Half-identical regions (HIRs) may be either identical by descent (IBD) (true matches) or identical by state (IBS) (false positive matches). Using phased data for the comparisons makes it much easier to determine which half-identical regions are identical by descent and identical by state.


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

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.

Tim Janzen presented a lecture on chromosome mapping at Rootstech 2014 entitled "Advanced Techniques for Use of Autosomal DNA Tests to Break through Genealogical Brick Walls". The presentation can be downloaded from Tim Janzen also presented a lecture on chromosome mapping at the Family History and DNA Day seminar held in June 2013 in Burbank, California. The Powerpoint slides can be downloaded here.

Kitty Cooper has provided a number of tools for mapping segments and chromosome mapping which are listed below.

Chromosome mapping with three siblings

Visual phasing is a methodology for assigning segments to specific grandparents based on the crossover points of three siblings. This technique can be employed when parents are not available for testing.

Blog posts and articles

Kitty Cooper's chromosome mapping tools

Blog posts and reviews relating to Kitty Cooper's tools

Other chromosome mapping tools

See also