Use GEDMatch ‘One-to-one’ comparisons

The GEDMatch ‘One-to-one’ compare page allows you to compare your results to one other person in the GEDMatch database.

Before starting this tutorial, you should have a GEDMatch account and have uploaded a DNA data file.

Step 1

Go to the GEDMatch website. Login to your account by entering your email and password and by clicking the Log in button.

GEDMatch Login
GEDMatch Login

Step 2

From the GEDMatch dashboard, click the ‘One-to-one’ compare link in the Analyze Your Data area.

GEDMatch: Select one-to-one compare
GEDMatch: Select one-to-one compare

Step 3

The next page is settings for the one-to-one comparison. In most cases, you should change two of the defaults. First, change the radio selection from Position Only to Graphics and Positions. Second, change the default “Minimum segment cM size to be included in total:” field to 10 (15 if you are Ashkenazi Jewish or another endogamous population).

GEDMatch: one-to-one compare 2
one-to-one compare settings

Step 4

The top of the one-to-one compare page has the key to the graphic (1),  the information on who is being compared (2), and the settings that you used (3).

GEDMatch - one-to-one compare report top information
Top information

Step 5

Matching segment information (1) shows for each chromosome. In the graphic, the non-matching, half matching, and fully matching SNP markers are on the top bar (2). The bottom bars shows where there are enough matching SNP markers to meet the cM threshold (3).

GEDMatch: one-to-one compare Matching information
Matching information

Step 6

The matching segment information table has the following fields.

  1. Chr – The numbered chromosome.
  2. Start location – The position on the chromosome where the matching segment starts.
  3. End location – The position on the chromosome where the matching segment stops.
  4. Centimorgans (cM) – The measurement of centiMorgins (cM) in the segment.
  5. SNPs – The number of SNP markers compared for the segment.
GEDMatch: one-to-one compare Matching DNA Segment table
Matching DNA Segment table

Step 7

When you share DNA from both your mother’s side and your father’s side with someone, you can have some segments that are fully identical. This is always the case with full siblings and double first cousins. It may also be the case when your parents are more distantly cousins.

You can find these areas on the one-to-one compare page by looking for green areas on the top part of the graphics.

GEDMatch: one-to-one compare Chromosome with full identical regions
Chromosome with full identical regions

Below, the graphic is annotated to show areas that don’t match (1, 5, and 9), areas that half match (2, 4, 6, and 8), and areas that fully match (3 and 7).

GEDMatch: one-to-one compare Annotated Areas
Annotated areas

Step 8

At the bottom of the one-to-one compare page, there is a summary of the matching between the two people.

GEDMatch: one-to-one compare - Summary information
Summary information

In the next section, I will cover using the GEDMatch X ‘One-to-one’ page.

1 thought on “Use GEDMatch ‘One-to-one’ comparisons”

  1. Any chance you could update this to explain the (slightly) different results given by the genesis.gedmatch.com version of the tool? E.g. what does “Total Half-Match segments (HIR)” mean, is “shared segments” the same as “matching segments”, and what does “## Pct SNPs are full identical” mean?

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