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cM Explainer™ – New MyHeritage Relationship Prediction Tool

Roberta Estes

At RootsTech, MyHeritage introduced cM Explainer, a new tool for all of their DNA customers that utilizes both the total matching cMs (centimorgans) plus the ages of the people involved, if provided by the customers, to estimate the relationship possibilities between two matches.

According to the MyHeritage blog article, here:

DNA Matches are characterized by the amount of DNA shared between two individuals, measured using a unit of genetic distance called centimorgans (cM). cM Explainer™ is unique in the way it uses both the centimorgan value as well as the ages of the two individuals (if known) to fine-tune its predictions, making MyHeritage the only major genealogy company to offer relationship prediction at this level of granularity and accuracy.

cM Explainer™ is fully integrated into the MyHeritage platform to shed light on any DNA Match found on MyHeritage, and is also available as a free standalone tool to benefit individuals who have tested with other DNA services.

Using cM Explainer

cM Explainer is automatically implemented for all MyHeritage DNA customers, so there’s nothing to do except utilize the tool in conjunction with the additional DNA tools already provided by MyHeritage.

Just click on DNA Matches if you’re a DNA customer, or under DNA Tools if you want to use the cM Explainer standalone tool.

Let’s look at my matches.

As you can see, two of my matches have provided their ages which appear in their match information.

The new cM Explainer probable relationship is listed as well. In Charlene’s case, she’s predicted to be a half first cousin, and Cheryl is predicted to be either a half first cousin, or a parent’s 1st cousin, which is another way of saying first cousin once removed.

Recall that “once removed” means one side of the descendant tree is one generation longer than the other. Cheryl and my Mom are first cousins (1C) and Cheryl and I are first cousins once removed (1C1R) or, said another way, I’m Cheryl’s first cousin’s daughter.

Probable Relationships

Some matches receive two listed “Probable Relationships,” but everyone can view additional estimates.

Click on the purple “Review DNA Match” button to view detail information.

My match’s segment information is provided, in addition to the possible relationships, in order of most probable first. To see additional information, click on the “show more relationships” link.

Charlene has a total of 5 possible relationships listed, each with its own probability calculated. One of my matches has a total of 8 possible relationships displayed.

Next, you’ll see the diagram of possible relationships.

Don’t forget to click on the “Relationships” dropdown in the upper right corner of the diagram.

You can click on Full relationships, Half relationships, or All Relationships.

I clicked on “all” which displays everything together.

Clicking the “Show probabilities for MRCA” box in the upper left-hand corner adds the probability that you and your match descend from a specific generation, or MRCA (most recent common ancestor.)

The highest or best probable relationship for cousin Charlene is calculated as 51.8% half first cousin.

The other possibilities are less likely. The second most likely is “Parent’s first cousin,” at 24.3%.

Charlene is my first cousin once removed (1C1R,) at the bottom. Stated another way, Charlene is my first cousin’s child, calculated at 4.5%, which should be genetically equivalent to a half first cousin at 51.8%. As you can see in the chart above, there’s VERY large probability difference between those two, which may be because of the expected comparative ages of the people in those positions involved.

Let’s take a minute to look at how half and “removed” relationships work genetically.

Half and “Removed” Relationships

In this example, John was married twice, to Mary and Sue. John had son Jim with Mary, and both daughters, Anna and Bonnie, with Sue. Their descendants took DNA tests.

In this chart, you can see that half-relationships of any kind carry half the average expected shared DNA as the full version of the same relationship. The yellow people, descendants of John and Mary, are half relationships to the green people because John was married to both Mary and Sue, having children with both wives.

The green people descend from full siblings, Anna and Bonnie, the children of Sue.

In the first generation, Jim and Anna are half siblings and share about 25% of their DNA. Anna and Bonnie are full siblings and share about 50% of their DNA. By extension, of course, Jim and Bonnie are half siblings too, sharing approximately 25% of their DNA, but not the exact same DNA as Jim and Anna share.

In the next generation, Jordan and Andrew are half first cousins and share about 6.25% of their DNA, while Andrew and Brad are full first cousins and share about 12.5% of their DNA.

Below the second cousin level, some cousins won’t match each other, but that doesn’t mean they aren’t cousins. It only means they didn’t happen to inherit a common segment of DNA from their common ancestors.

At the fourth-generation level, Jeremy and Abraham are half third cousins and share less than 1% or about 26.56 cM of their DNA, while Abraham and Betty are full third cousins and share about 53.13 cM of their DNA.

That half division of DNA occurred several generations earlier because Jim and Anna are half siblings which means that they only share half as much DNA as full siblings Anna and Bonnie. Of course, each subsequent generation will be a half relationship, and share roughly half as much as the full equivalent of that same relationship.

Once Removed

However, when the generations are offset by one, or once removed (1R,) the DNA is halved again. Looking at the chart again, half third cousins (3C,) Jeremy and Abraham share about 0.39% or about 25.56 cM of their DNA. Abraham and Beverly, who are 3C1R (third cousins once removed) are ALSO expected to share about 25.56 cM, the same amount of their DNA. In this comparison, the halving occurs in the last generation by the generational offset, when comparing Abraham with Beverly.

Of course, Jeremy and Beverly share the smallest percentage of all, because they are Half third cousins once removed, so they would be expected to share only about 13.28 cM of their DNA, assuming they share any at all.

I wrote about the various percentages expected of each relationship level and compiled a comprehensive chart in this article.

Of course, MyHeritage has included the factor of age to attempt to refine the relationship more succinctly.

How Accurate is cM Explainer?

I created a chart of my closest matches who are known, proven relatives.

Results where My Heritage has provided exact, accurate predictions are shown in red.

My first thought when I saw this new tool was that all of the people with whom I shared a Theory of Family Relativity (TOFR), especially relationships I had confirmed (or at least not rejected) would be predicted in cM Explainer to be that same relationship. Well, I was wrong.

Of the 8 matches with whom I have an accurate TOFR, the relationships of 4, or 50%, are correct, but the other 4 are not, so clearly, MyHeritage is not relying on TOFRs for cM Explainer, at least not solely, if at all.

Person Total cM # Segments Actual Relationship TOFR MyHeritage cM Explainer
Michael 822.8 31 1C Y 1C
Alberta 744.2 25 Half niece *1 (2nd on list) Y 1C
Dana 521.8 17 Half 1C1R (not on list) N 1st C dau, half 1C
Charlene 477.5 24 1C1R *2 (4th on list) N Half 1C
Cheryl 477.2 23 Parent’s 1C (2nd on list) N Half 1C, parent’s 1C
David 460.4 17 2C (3rd on list) N Half 1C
Buster 409.9 16 1C1R *3 N Parent’s 1C
Donald 381.7 17 1C1R (3rd on list) N 2C
Kurt 378.9 16 Half great-nephew (not on list) N 2C
Teresa 330.4 13 Half great-nephew (not on list) Y 2C
Shirley 223.3 8 2C1R (2nd on list) Y 2C
Sydney 217.8 10 Half great-nephew (not on list) N 2C dau, 1C dau
Buzz 212.7 9 2C Y 2C
Amos 182.7 8 1C2R (8th on list) N 2C son
Denny 166.9 6 3C (2nd on list) N 2C
Thomas 156.4 7 2C Y 2C
Patty 150.6 9 2C Y 2C
Cathy 102.9 5 3C Y 3C
Carol 87 7 2C1R (2nd on list) N 3C

*1 – Half aunt/uncle is equivalent to half niece/nephew – it’s simply a matter of perspective.

*2 – 1C1R (first cousin once removed) is the same relationship as a first cousin’s child, just said differently.

*3 – Your parent’s first cousin in your first cousin once removed (1C1R.)

In the Actual Relationship column, I’ve indicated the actual relationship, then if the actual relationship is shown on the chart of possibilities provided by MyHeritage, and if so, at which position.

For example, I’ve listed Alberta’s “Actual Relationship” as my half-niece. Additionally, there’s a comment at *1 below the chart. MyHeritage predicted Alberta as my first cousin, but the correct half-niece designation is shown second on the list.

In this case, I’m fairly sure I know exactly why the relationship miscalculation occurred. Alberta’s mother, my half-sister, was born to my father’s first wife. My mother was 22 years younger than my father, so my mother is roughly the same age as my half-sister. I am the same age as my half-sister’s oldest children. Therefore, we have an unusual generational difference where ages might be misleading.

In the second position, MyHeritage estimated Alberta as half-aunt, which is the same as half niece, depending on whose perspective you’re speaking from, so cM Explainer was close. In normal circumstances, 1st Cousin is probably the most likely relationship although having children separated by two decades certainly is not unheard of.

MyHeritage correctly predicted 6 of 19 relationships, for 31.6% accuracy.

The correct relationship was on the relationship list most of the time, but was omitted entirely 4 times. The common factor in the entirely missing relationships is that they are all half-relationships. While they were not all from the same family line, they did all involve long generations, meaning children born over a very long period. That’s not uncommon with half-siblings, and half relationships are notoriously difficult to sort from other candidate relationships. These situations might possibly be considered statistical outliers.

Equivalent Relationships

A half first cousin should be genetically equivalent to a first cousin, once removed, based on the amount of expected DNA for those relationships.

However, in at least one case, these two relationships are calculated with different resulting probabilities. Half first cousin is 41.5%, and 1st cousins child (aka 1C1R or 1st cousin once removed) is 43.8%.

Keep in mind, though that MyHeritage is using the age of the two individuals in their calculations, which could alter the results based on the combination of factors calculated into the probabilities. It’s 85% likely that the match is one of those two relationships.

Your Thoughts?

I’m interested in your thoughts on this new tool. How does it work for you? What about endogamy or pedigree collapse? Do you find it useful? How are you utilizing it in your research?

Shortly, I’ll do a comparison article with other tools to see how the publicly available cM Explainer tool stacks up against the rest.

Thanks to MyHeritage for making this tool free for all to use, here.

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