Understanding Half Relatives - Genetic Genealogy

Half-siblings are pretty straight forward. But half-relatives can confuse most genetic genealogists when they review DNA matches.


Half relationships can be confusing for some people. I know it was for me and still. I usually have to draw the half-relationships out to understand what I am talking about. So let’s talk about those people we are half-ly related to.


Video: Understanding Half Relatives - Genetic Genealogy

Watch this video.


Growing up, I had friends who were part of blended families. They were step-siblings. Step-siblings are not genetically related, but their younger siblings were related to both of the step-siblings as half-siblings. That’s a mouthful, So let me try and use some pictures to explain it a little better.


This is one time where the video is better than the blog post. Check it out.


Everyone has two biological parents, a mother and a father. We get half of our DNA from each one, so we are related to each. Our full siblings with the same parents are then fully related to us because we are ½ related through our mother and ½ related through our father. If one of our parents had children with another person, they would be our ½ sibling because we are related through 1 common parent, not 2.



chart of step-siblings


So an easy way to look at this is a half relationship is related through ½ of the most recent common expected pair of ancestors.


So half cousins - well, the common ancestor pair for cousins would-be grandparents. So half cousins only share one of their grandparents in common. In other words, the children of half-siblings are half cousins.


That can seem pretty simple, but it can really get confusing when you go back a generation. Take a look at this. We have our half-siblings. They share one parent.



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Relationship Between Half-Siblings and Their Aunt?


What is the relationship between each of them and the children of their aunt? Half-cousin or cousin or something else?


How are these half siblings related to an aunt's child?


It turns out they are cousins, because cousins share a common set of grandparents, the relationships between half-siblings parents don’t affect their cousin relationships. This extends through all generations.


You can be half 1st cousins with someone, but both of you be full 2nd cousins with a third person because you share the same great-grandparents.


With DNA, half relationships are easy in theory but can be complicated in identifying unknown relationships. Half relationships share half of the expected amount of DNA as full relationships. So full siblings share 50% of their DNA, half-siblings only share 25% of their DNA. Half siblings also wouldn’t share any fully matched segments.

Chart of Centimorgans shared by first cousins and half first cousins

Half 1st cousins share 6.25% of DNA, while full 1st cousins share 12.5% of DNA. In a perfect world where averages were the rule, these relationships would be easy to distinguish by DNA. But DNA doesn’t follow the average, so a Half 1st cousin could share as much or, in some cases, more DNA than a full 1st cousin.


And this is the real trick in DNA research or unknown relationships. Two people may share DNA at the 1st cousin or second cousin level, indicating that they share common grandparents or great-grandparents. But comparing their family trees shows no overlap. There must be a mistake in the family tree. There is likely a half relationship in there that is unknown in the family. And that is where knowing where people were at certain times is important to identify where the half relationships are.


Until the late 70s, the biological mother and father had to physically interact 9 months before the child's birth. In the last 40 years, there have been thousands of sperm and egg donors who have children that are now or almost will be adults. If they take a DNA test, they may discover half-siblings.


One thing that DNA testing has shown is something population scientists have suspected all along. There are a lot of half relationships out there. Most of them are unknown and waiting to be discovered.


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