8

This is an advanced question. I know how to use the tool. A good answer will explain how it does what it does. An ideal answer would come from the person who wrote the tool for GEDmatch.

The purpose of the tool is to answer the question: “How might these two people be related?”

The Relationship Tree Projection tool is only available to “Tier 1”, paid members of GEDmatch. (Currently, $10 per month).

The tool is only useable in certain circumstances. First, you must have two people whose results are on GEDmatch. Second, they must have a match in their X-DNA, not just autosomal. Third, you must know the ages of the participants, or at least their relative ages. You will also want to know the genders of the two participants, but I believe this is determined automatically.

When you run the tool, it suggests a small number of family trees showing exactly how the two people might be related. That’s not so surprising. What is surprising to me is that it shows the gender of everyone in the tree. For example, it might suggest not only that your match is your 1st cousin, but that they are the child of an aunt, not an uncle.

In the case where I used this, it suggested that the matches were 1st cousins 2x removed, through a particular tree of males and females. This was the end game of a search for someone’s biological father. When we followed that diagram, it led to someone who we now know was in fact the father.

The tool eliminated many other possible cousinship relationships we would have otherwise had to check.

Are the results shown by the tool merely the most probably family trees, or does it know that the trees it doesn’t show are impossible?

7

The GEDmatch Relationship Tree is a wonderful idea the imaginative people at GEDmatch put together. No, there is not much documentation about it and it is still said to be a "work in progress", but it has been there for a number of years and does something few if any other DNA analysis tools do.

It uses the two people that you give it, whose relationship you want to determine. It uses both the autosomal sharing levels and X-DNA sharing levels to determine relationships.

It starts off by building up the tree for the first person. It adds the father and mother. Then it adds their grandparents. Then their great-grandparents and it keeps going as far as it needs to. When it adds each pair of parents, it add a father and a mother, so it knows their sex as it adds them.

Then for each ancestor, it determines the average autosomal shared and the average X shared with the original person. For autosomal, it lists parents as sharing 3587 cm, grandparents half of that (1793.5), great-grandparents half of that (896.75), etc.

For the X chromosome, it uses X-DNA rules. It starts with the first person having 196 cM. If the first person is male, then his father shares 0 and his mother shares all 196 cM. If the first person is female then each of her parents share on average half or 98 cM each. It continues this pattern up the tree depending on whether the person is male or female.

Once it calculates the autosomal and X the first person shares with an ancestor, it then takes the ancestor, and works its way back down to get to the second person. It asks you for the generational level of the 2nd person relative to the 1st person so that it knows how many generations to go down.

And it does the same thing in reverse. For every ancestor, it adds a male child and a female child. For every child, it calculates the autosomal as half shared of the parent.

For each female child, the X is calculated as half the parent. For each male child, the X is calculated as 0 if the parent is male, and the full value of the parent if the parent is female.

Once it reaches the 2nd person's generational level, it will then compare the autosomal shared and X shared with the amounts shared that you entered as input. If the values are within the "overlap factor", then that relative is considered a possible candidate. The default overlap factor will include anyone where your entered values are between 29% below the projected value or 41% above the projected value.

It will then draw the relationships it finds in a very nicely done chart that contains all the average shared matches for autosomal and X that it calculates.

If no relationships are found, you can increase the overlap factor. An overlap factor of 2 goes from 50% below to 100% above.

Below is an example of a run I did, where I compared myself (male) with an female DNA tester that I don't know how I'm related to, but we share 41.9 cM autosomal and 27.9 cM on our X. Level 1 didn't give me any results, so this is run with level 2 which gives two most-likely relationships.

Because of the variation in DNA that descendants get from their ancestors (it's not exactly 50% except for parent-child), the relationships given are in no way guaranteed, but they are the ones that are quite possible and are most likely as far as what the averages say.

In my case below, the tool tells me that the most likely ways I relate to this person as a 3rd cousin are through the following two connections:

  1. My mother's father's mother's sister's son's daughter's daughter.

  2. My mother's father's mothers' sister's daughter's son's daughter.

Both give expected shared autosomal of 56 cM and X of 18 cM compared to the 42 cM and 28 cM actuals.

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  • Thanks for a good discussion. I'm going to disagree with "It will then draw the relationships it finds in a very nicely done chart..." . I found the chart bizarre and difficult to understand. I did manage to understand it well enough to find it useful, but it is in a totally weird, non-standard format. Do you know what the I= and J= items on the chart mean? – Jamie Cox Jan 25 '18 at 0:34
  • @Jamie - Why do you find it weird and difficult to understand. Yes it is rotated 90 degrees to get it on the page better, but those are the lowest generations on the left (including you and the possible locations for your cousin), and your common ancestor on the right. The I and J just look like the row number and column number where the box is placed. They really need not have included those. They could better have replaced them with the relationship of each person to yourself, e.g. ggg-grandfather, or 2nd cousin twice removed. – lkessler Jan 25 '18 at 1:48
  • The chart seems severely compressed left to right as though afraid they would run out of paper, or to make it fit on a phone screen. What is going on with the weird jog in the line between I=8, J=8, and I=13, J=10 ? Does that signify something? This chart really doesn't look like any other genealogy chart I've ever seen. I just want to say for the record, that this is a fantastic, unique tool, and totally worth further development. – Jamie Cox Jan 25 '18 at 2:08
  • I'm going to accept this answer. I learned most of what I was looking for. Is there any official documentation on the Relationship Projection Tool? I'm surprised GEDMatch hasn't given it more of a write-up. – Jamie Cox Jan 25 '18 at 2:16
  • @Jamie - The graphing tool they use could definitely be better. There should not be any breaks in the lines and no, that jog needn't be there. There's no official documentation that I was able to find. See my blog post where I have a bit more about it: beholdgenealogy.com/blog/?p=2417 – lkessler Jan 25 '18 at 2:28

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