“Low sharing” is a term often used by genetic genealogists to describe an amount of DNA that is towards the low end, or even lower than the bottom range of the typical amount of shared autosomal DNA for two people for a given relationship. In this post, you’ll learn:
- About how the term “low sharing” is used
- See an example of low sharing family members
- Find out possible effects of low sharing on DNA inheritance
- Learn why not everything can be explained by “low sharing”
Note: “Low sharing” is a term used by geneticists (which I am not!), but the way that it is used in the genetic genealogy field is different than the way that scientists would generally use it.
More about the term “low sharing”
If you are new to genetic genealogy, then you might be interested in a little more explanation about how and why the term “low sharing” is used. You might already know that there is normal variation in the amount of shared DNA that is typical for any given relationship. For example, second cousins generally share between 70-350 centimorgans (cMs) of DNA. For every type of relationship, there is a range like this. The closer the relationship, the higher the amount of centimorgans, but the wider the range, with one exception. Parent/child relationships will have a very high amount of shared DNA with little variation.
Occasionally, we will see matches that fall at the very bottom, or even below the range that is expected. With the example of second cousins, you would expect shared DNA to fall usually somewhere in the middle of the range. If you only shared 85 cMs, or even 60 cMs, then you might say that you are low sharing with your second cousins.
How do you know if you are low sharing, are related in a different way, or have a “half” relationship?
It is important to mention that there are average ranges of shared DNA for relationships for a reason. This is because most relationships of a given distance will fall within the typical range of shared DNA for that type of relationship. The practical application of this for those of us who are looking at a DNA match and trying to determine their relationship to us is that if we share less DNA than expected, then we should first consider the following:
- We are related in a different way to the match, meaning that if they don’t share enough DNA to be a second cousin, we should consider that they are a third cousin, etc
- They might might have a “half” relationship to us. For example, if they share less DNA than the bottom range for second cousins, is there a possibility that they are half-second cousins?
Unfortunately, the only way to know for sure whether someone is low sharing with you, has a half-relationship to you, or is related in a different way, is to have all of the involved parties do a DNA test. For example, if I have a second cousin who I believe might be low sharing with me, then I should try to have my parent and my second cousin’s parent do a DNA test to see if their DNA falls within expected range. If I could test the relevant grandparents, that would be even better.
This isn’t always practical, since sometimes the people who could really answer our questions are no longer with us. A second option would be to have as many of your siblings, or your parents’ siblings, do a test as possible, since there will be different amounts of shared DNA between your siblings (or aunts/uncles) and your DNA match, and you might be able to narrow down possibilities with additional data points.
Are you wondering what I mean by “involved parties”? In the case of a second cousin, you would need to be 100% sure that both of the children of the the great-grandparents are full siblings, since one explanation for a low sharing second cousin match could be that the grandparents of the second cousins were half-siblings and not full siblings. This can be determined by testing the siblings. Additionally, if the children of the siblings share within the range of full first cousins, then this can also serve as “verification” of a low sharing second cousin match. If the first cousins don’t share within the range of full first cousins, you still can’t be sure, since they could also be low sharing first cousins (see below for an example).
Example of two first cousins who are low sharing
We do occasionally have access to enough information to help us be sure that a DNA match really is low sharing, and that there is no other explanation for the low amount of shared DNA between two family members. I have a real-life example for you to help illustrate the concept, and to help provide “proof” that this is, in fact, possible:
My father has a first cousin. Let’s call him “Sam”. On my father’s DNA match list, Sam shows up as a second cousin, even though I know that he is my dad’s first cousin. They only share 570 cMs of DNA. The typical range of full first cousins is about 575-1330, and the range for half-first cousins is 215-650.
If I didn’t know better, I could easily assume that my grandmother and my great-uncle are half-siblings. You can see that the range for half-first cousins overlaps with the shared DNA for full first cousins, and would include the number of shared cMs between my father and Sam. Despite this, I know that my dad and his first cousin are low sharing first cousins. How can I be 100% sure of this? It’s as easy as DNA.
My grandmother, her brother, all of her brother’s children, as well as my father have done DNA testing with the same company. Their DNA results give me four pieces of “evidence” that my dad and his first cousin are low sharing, and not half-first cousins.
- My grandmother shares 2648 cMs with her brother, which is well within the accepted range of full siblings
- My grandmother shares 1592 cMs with Sam, which is exactly where you would expect to see a full aunt/nephew relationship to be
- My father shares 1633 cMs with his uncle (my grandmother’s brother), which is practically in the middle of the range for full uncle/nephew
- My father shares 893 cMs with Sam’s brother, which is just about in the middle of the range for full first cousins
How did this happen? DNA is passed down in a fairly random way. My grandmother and her brother share a fairly normal amount of DNA for full siblings, so we can’t write it off as the fault of low sharing siblings. My dad and Sam inherited 50% of their DNA from full siblings, but the genes that they inherited just happened to have less segments that match each other.
Possible effect of low sharing on DNA inheritance?
I haven’t seen any studies of generations of people descended from two low sharing family members that could tell me conclusively whether or not you should expect that their descendants be low sharing, too. In my own family, I have found that even though my dad is low sharing with his first cousin, his mother was not low sharing with her brother. I am not low sharing with Sam (he is my first cousin once-removed and we share 312 cMs), and my daughter is not low sharing with Sam, either (they are first cousins twice-removed and she shares 119 cMs).
I hope that this post helped you, and provided some insight as to what low sharing is, that it is possible, and how to figure out if this is what is going on with you and your DNA match. If you have any questions about something that you read here or would just like to add your experience with a low sharing match, I would love to hear from you in the comments.
Thanks for stopping by!