How Do I Figure Out if I Have a Full or Half Relationship with a DNA Match?

If the descriptive relationship terms full or half are unfamiliar to you or find it a challenge to describe relationships, this blog post may help you. Figuring out the exact relationship you share with a DNA match can be challenging at times. When the relationship is a “half” relationship, it can be even more tricky. The amount of shared DNA between two matches doesn’t directly indicate whether a relationship is full or half. Traditional documentary research is needed to definitively discern if a relationship is full or half.

It’s all about the common ancestor

It is essential to understand and name half relationships in a family tree because it helps you build an accurate pathway to the common ancestors you share with your DNA matches.

When we work with DNA in our family history research, we spend large portions of our time figuring out the ancestor or ancestral couple we share with each DNA match. We inherit DNA from our parents, who inherited DNA from their parents, and so on, back in time. The DNA we share with our DNA matches was passed down from one or more ancestors that we share in common with our DNA matches.

We seek the common ancestor(s) shared with our DNA matches for several reasons. We may wish to identify a common ancestor on the other side of the proverbial brick wall so that we may extend our ancestral lines. We may hope to learn more about common ancestors, and our DNA matches may have documents, photos, memories, etc., about the ancestors that we don’t have. We may be building our family tree. The family trees attached to our DNA matches’ profiles can give us clues about ancestral connections that we can confirm through our documentary research.

You can determine full and half relationships by identifying the common ancestor shared with a DNA match.

– If you share just one common ancestor with a DNA match –one parent, one grandparent, one great-grandparent, etc., then you have a half relationship.

If you share an ancestral couple with a DNA match – two parents, two grandparents, two great-grandparents, etc., then you have a full relationship.

 In a half relationship, two individuals share one parent or grandparent, etc., instead of sharing two parents or grandparents, etc., in a full relationship. Half-siblings share one parent, and half first cousins share one grandparent as a common ancestor instead of two grandparents in a full relationship.

This situation may occur if a couple has a child, then one of the spouses dies, and the surviving parent marries again and has another child. The children in that family are half-siblings. Another example of a half relationship occurs when a parent has a child out of wedlock and then has a child or children with a spouse. The first child is a half-sibling to the other child or children.

In a half relationship, you share half of the DNA amount expected in a full relationship. This is because only one of the common ancestors shared by both you and your DNA match contributed DNA to you both.

When a person has one or more children with one person and has one or more children with another person, the children descended from the two different relationships are half-siblings. The children share DNA with the parent in common and with each other, but they have 50% of their DNA from their other parent that they do not share.

Examples of Relationships

In this example, Bill was married two different times. The children he fathered with Angela are full siblings, and the children he fathered with Caroline are full siblings. The two sets of full siblings from each spouse are half-siblings to each other. Half relationships are shown between Bill’s children with Angela and his children with Caroline.

This next image illustrates the grandchildren of Bill from above and the relationships they share with each other. Full relationships are labeled for descendants who share the ancestral couple Bill and Caroline. Half relationships are shown for Diane and Victor, who share only Bill as a common ancestor with Caroline’s descendants. A Half-Aunt/ Half Niece relationship between Diane and Wanda indicates that they share just one common ancestor – Bill, as they don’t share a mother/grandmother.


The following image shows Bill as a great grandparent when another generation is added. Bill’s great-grandchildren are either half or full 2nd cousins depending on if they share a relationship with Bill and their great-grandmother, Caroline. If they don’t share Bill’s first wife, Angela, the cousins are half 2nd cousins.

When another generation is added to the family tree, Bill becomes a great-great-grandfather. The youngest generation shares either full or half 3rd cousin relationships depending on if they share Bill and their 2nd great-grandmother or not.

Next, let’s take a broader look at Bill’s brothers, Thomas and Samuel, who also descend from both Benjamin and Bonnie. The relationships between Thomas and Samuel’s descendants (shown in the outside rectangles) are full, not half, relationships to all of Bill’s descendants. The relationships are classified as full relationships because the common ancestors they share are an ancestral couple. Both Benjamin and Bonnie passed DNA on to their children Bill, Thomas, and Samuel, who passed it on to their children.

The most important concept to remember when discerning between full and half relationships is to see if you share one ancestor or an ancestral couple with a DNA match.

– If you share just one common ancestor with a DNA match – one parent, one grandparent, one great-grandparent, etc., then you have a half relationship.

If you share an ancestral couple with a DNA match – two parents, two grandparents, two great-grandparents, etc., then you have a full relationship.

As you incorporate DNA results in your family history research and discern whether you share a full or half relationship with your DNA matches, you will be able to compare the amount of shared DNA in the Shared cM Project. Enter the amount of DNA you share with your DNA match to see if the relationship you have identified fits within the expected amount of DNA for that relationship. This step is a good check to see if you are on the right path in assigning a specific relationship to your DNA match.

Assigning specific relationships to your DNA matches helps you put them in the correct place in your family tree. It also enables you to discern how much ancestral DNA you may share with a DNA match, and it can help you make progress in your family history research.

To learn more about using DNA in your family history research, see our new book, Research Like a Pro with DNA, Kindle version. The print version will be published soon!

We are excited to have you join us on the journey to Research Like a Pro with DNA!

Source: Family Locket

Posted On: April 17, 2021 at 08:05PM