The Shared CM Project
Although siblings always inherit half of their DNA from each parent, it's a randomly different 50%.
Each sibling inherits some of the same DNA as another sibling as well as some that the other sibling did not. A child can't inherit any DNA that their parent did not inherit first.
This is why it's so important to test as many members of an older generation whenever possible. They carry twice as much DNA from a particular line than any of their children do.
If a parent isn't available for testing, try to test yourself plus as many aunts and uncles as possible.
Full siblings have the same genealogical relationship to any other member of their family.
However, they will share differing amounts of DNA with their cousins and other relatives - because they inherited different parts of their parents' DNA.
We measure the amount of DNA shared between any two relatives in centiMorgans (cM).
For every relationship, we know the average amount of DNA two people will share - along with the possible range of values from low to high.
Thanks to the work of genealogist Blaine Bettinger and many volunteers with the Shared CM Project, we have a good understanding of the range of DNA amounts
to expect for any given relationship.
Shared CM Table
As of June, 2017, here is the current table of shared CM amounts for most relationships.
Note that for reciprocal relationships (like grandparent and grandchild), it isn't possible to tell which person is which from the amount of DNA shared.
Notes:
- AUNN is my acronym for Aunt / Uncle / Niece / Nephew
- A first cousin once removed is the child of your first cousin - or the first cousin of your parent
- A third cousin twice removed is the grandchild of your third cousin - or the third cousin of your grandparent
How much DNA do you share with a relative?
If you've tested with AncestryDNA, here's how to find out how much DNA you share with your matches.
Further Reading
Do a quick web search of the "Shared CM Project," or visit: