Assigning a Side & Marking an MRCA

Assigning a side to a match can occur at any point of the analysis process. The earlier you can identify a maternal or paternal side for a match, the sooner you can start to narrow down the range of potential data to work with.

Marking an MRCA is typically the final step of the research process, however some people can mark an interim MRCA person that may assist them with their research and analysis.

Side designations are universal throughout GDAT
M - Maternal
P - Paternal
B - Both
I - Identical by State / Identical by Chance / Identical by Place or Population

Assigning a Side

Knowing if a specific match is maternal or paternal can prove helpful with the overall analysis of surrounding matches. Sometimes being able to identify the side for a match is simple because you'd been fortunate enough to have tested a close family member on one side of your family, such as a parent, an aunt or uncle, or a 1st cousin. Sometimes it's not so easy to identify because our close genetic relatives are unknown to us, or have already passed.

The potential methods used to identify which side a match occurs on will depend on your own situation, and will not be covered here.

Setting a Side from the Chromosome Tab

Click on the ? associated with a segment for a known maternal or paternal match, and enter M, P, B, or I accordingly.

It is also possible to assign a Group name to this segment by clicking in the appropriate space under the Group heading and typing.

Setting a Side from the Relative List view

Manual entry of a side is done by clicking on the ? and replacing it with M, P, B, or I accordingly.

The Right Click (Control-Click on Mac) menu on the Relative List has the option of “Set the Side Values”. This will “pull” any values you’ve set elsewhere for those matches, but this will overwrite any previous set side allocations.

A Family Group can be assigned here by typing clicking the Family Group field to enter information.

Setting a Side from the Relative's Notes area

In the Relative's Notes view, side is set by selecting one of the radio buttons available.

A Family Group can be assigned here as well, either by typing directly into the Family Group field and pressing enter, or by selecting one of the options potentially available in the drop down menu, depending on what family Group names already exist in the database.

Any side designation, and/or Family Group entry here, will "push" through to the Relative List.

Setting a Side from the Relative's DNA Comparison area

For this process, we work from the DNA Comparison view for your match, and we disregard the name in the profile selection box (1). Instead, we focus on the Profile person listed for each segment in the DNA Segments box (2) and how that person relates to the relative (3).

For each segment, ask yourself “Which parent does the line of the family tree go through from this segment owner to get to the MRCA for this relative?”

If there is no endogamy or pedigree collapse involved, use the Right Click menu to assign information to more segments and one or more profiles if desired.

Note: watch who is the segment owner that you have selected on the left (2), especially if you have multiple generations within your profiles. Not all paths will be the same.


  1. If the Relative is the descendant of the Profile person, e.g. a child or grandchild, the side is marked as Both.

  2. If the Relative is the full sibling of the Profile person, the side is also marked as Both. The child or grandchild of a full sibling is also marked as Both.

  3. If the relative is a half-sibling, the shared parent is the side that is marked.

Using an Interim Group name or MRCA

While GDAT has been developed and designed to work best with known MRCA's and associated group names, there are ways to have the program help you with your research by the careful use of interim MRCA's and group names. How you do your research and record your interim results is entirely up to you though, and the use of interim names and/or MRCAs is not a necessary part of the process. The Relative Notes view, along with the Document Ancestor Research and Parental Confirmation areas should provide enough for the majority of people. However, the following has been provided as an example of how the use of an interim MRCA might be used.

In the example shown, I can see that KP and the match have a number of ICW relatives in the same location. Three of these relatives are known in their relationship to KP, and the MRCA note entered, quickly shows me which branch of KP's tree the relative connects on.

At this stage though I do not have enough clues or information to assign an actual MRCA to this relative, but I have two options on how to mark this relative going forward.

One option available, is to mark the Side, and assign a group that is derived from but not identical to the closest known groups of the Triangulated / ICW relatives.

In this case, the closest known groups are both "Tertius-Quartus" as I have recorded the MRCA couple in the MRCA note as this couple.

Thus I could choose to mark the currently unknown relative with the group name of "Tert-Quar Anc" - indicating to me that this relative is connected somewhere along the Tertius-Quartus branches of the tree.

Another possible option, would be to use a more distantly connected but still known triangulated relative, and base the interim group name on this information.

In the example shown, Septimus-Caelius is further up the Tertius branch, and is not connected to the Quartus branch, and as such, we could either fine tune the interim group name to indicate a Tertius ancestral connection, or we could further fine tune the grouping and use "Sept-Cael Anc".

In both of the above examples, we have simply assigned an interim group name to one or more segments associated with a match, and this group name will appear on the Chromosome browser associated with this relative and these segments.

You may of course choose another method of marking interim information, by using triangulation group numbering or another personal method of your choosing. Whatever interim method you choose to use, it is suggested that you record notes about your reasoning in either the Research Notes section or within the MRCA Notes section, or even in both places.

If we want to see this segment in the Segment map however, we will need to take this one step further and Mark an MRCA.

Marking an MRCA

The key to marking MRCA's in GDAT is to select the child of the MRCAs from whom the DNA is inherited by the segment's profile person. This then automatically sets the correct group name and side for the segment.

Take a look at the examples below.

In this first example, the relative is the segment profile person's 1st cousin, and the person to mark as the child of the MRCA, is the profile's parent that is the sibling to the relative's parent.

This assigns the appropriate grandparent group to the match, and also automatically assigns the correct side.

In the next example, it was determined that this person was descended from one of the other children of S Melitta and S Psittaci.

The segment's profile descends from their son W Melitta, and as such, W Melitta is the person assigned to the MRCA position, enabling the Melitta-Psittaci group name to be correctly assigned to this segment. Again, the side is assigned automatically.

What about a half-cousin connection?

A relative who shares just one ancestor, will have that ancestor assigned as the MRCA, enabling for a more accurate labelling of the source of the shared DNA via their parent's group name.

Note in the adjacent example that E J Limonata's partners (who appears as the Ahnentafel number immediately before her) are different in each tree.

A match that is technically a 3/4 relationship, that is, a man had children with sisters, or a woman had children with brothers, is marked in the same way as a full relationship match. Use the child of the pairing that the segment profile person descends from.