How to ascertain the degree of kinship from the amount of shared DNA? cM Project tool

by Marta Sobota

After receiving your DNA test results we can see the list of matches, ie. people we are related to. How to decipher these numbers?

On our matches list, we have the amount of shared DNA given next to each person. On most websites, this information will be given in centimorgans (cM), and on 23andMe instead of centimorgans we will see the percentage of shared DNA.

To understand what this means we can use DNA Painter Shared cM Project – a tool helpful by ascertaining, what kind of degree of kinship relates to a certain number of centimorgans (cM) or shared DNA percentages (for analysis we can use either of these measuring units)

Let’s see how it works: two people share 215.3 cM:

Sprawdźmy to na przykładzie: dwie osoby dzielą ze sobą 215.3cM:

215cm match
215 cM of shared DNA

We open the tool Shared cM Project and write 215cM:

Under the window, where we wrote ‘215’ we can see the option ‘or enter %’ in green – when choosing it we can instead of centimorgans write in the percentage of shared DNA (f.ex. if we did the test in 23andMe).

After entering the number of centimorgans we see a table with the most probable degrees of kinship with that person. In case of 215cM the table looks as follows:

Możliwe pokrewieństwa dla 215 centymorganów

We can see here, that the biggest probability (49%) is, that this person is one of the following relatives:

* Half GG-Aunt / Uncle – half brother/sister to our grandpa or grandma

* 2C – second degree cousin

* Half 1C1R – a child of a half brother/sister of our gandpa/grandma

* 1C2R – great grandchild of our aunt/uncle

* Half GG-Niece / Nephew – grandchild of our half siblings

Next we see, that a lower possibility, but still high as 40% is that this person is:

* Half 2C – second degree cousin from our grandpa’s/grandma’s half siblings

* 2C1R – a child of a second degree cousin or a parent of a third degree cousin

* Half 1C2R – a child of a half sibling to our great grandparent or a grand child of our parents’ half sibling

* 1C3R – in this case I’d recommend having a look at the table below to find this relationship 🙂

After entering the number of centimorgans we will see, that some fields faded, while others remained vivid. The vividly colored ones present the possible degree of kinship for this number of centimorgans.

Tabela możliwych pokrewieństw
Table of possible degree of kinship for 215 cM of shared DNA

Let’s analyse the relationships with the highest percentage of probability (49%). In the table below I have circled 5 possible degrees of kinship, given by the DNA Painter. We can see straightaway that the table contains not 5 but 7 blue circles. It is because 1C2R means first cousin once removed, so this person can be both:

  • a grand child of your first degree cousin
  • a grandma/grandpa of your third degree cousin

Tabela pokrewieństwa 215 cM
The most probable degrees of kinship for 215 cM

In the beginning, we look at the most probable degrees of kinship. Let’s think about the age of the person we are ‘trying’ to place in our genealogical tree.

Is it possible for the gap between us to be two generations?

In our case the person is 50-60 years old, so more or less the same age as the person who had the test done. So it is rather impossible for them to be a half sibling to one of our grandparents or a grand child of our half siblings. We can cross these options out:

Tabela pokrewieństwa pierwsze wykreślenie
Wykluczamy relacje, które najprawdopodobniej nie są możliwe, ponieważ obie badane osoby są w tym samym wieku

The same goes for the relationship described as ‘1C2R’ – we can also assume, that a person the same age as we are, will neither be a child of our grandparents’ sibling, nor a grandchild of our first-degree cousin. Of course, it is possible in some extreme cases but it happens very rarely and only when we can see in our genealogical tree that such a situation could have occurred (generation shift).

Consequently we also cross out these two options:

Tabela pokrewieństwa drugie wykreślenie
These relationships are impossible due to the age of tested people

We are left with three blue circles. One is for a second-degree cousin, and the other two are for a child of our grandparents’ half sibling or a grandchild of our parents’ half sibling. In this case, a difference in one generation, despite the same age, is far more possible.

In this case it turned out that the people are second degree cousins, who didn’t know each other due to after war emigration. So 215 cM is actually to the point with what is shown in the field 2C (229cM)

When I write a message to relatives I often attach a printscreen shot with possible degrees of kinship and I tell them about the Shared cM Project result to show them possible relationships. This makes it easier for the recipient to understand, what shared DNA means.

I use the Shared cM tool quite often when analysing DNA results. It enables me to quickly check, more or less where to look for our shared ancestors on my genealogical tree. I recommend to learn how to use it: DNA Painter Shared cM Project 🧬

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