Understanding DNA Relationships

Measuring DNA relationships is a key function of genetic genealogy. These DNA tests can confirm that two people have ancestors in common and predict the relationship.

By measuring how much DNA two people share, autosomal DNA tests can confirm or disprove any suspected relationship out to the second cousin level. And many more distant relationships will be identified, though not as precisely.

Even though the amount of shared DNA gets cut roughly in half with each generation, these autosomal tests are a great way for adoptees and others to confirm suspected relationships.

You can do as I did and confirm a child's father when that man is deceased (or unwilling to test). Just test someone who would only be a close relative to the child IF the suspected man is the father.

The autosomal DNA tests used for genealogy are in many ways more powerful than a paternity test. Besides confirming a parent-child relationship, these tests can identify full siblings, half siblings, aunt/uncle and nephew/niece, grandparent and grandchild, first cousin and second cousin.

Shared DNA on an autosomal test is measured in a unit called centiMorgans (cM). To see how much DNA is typically shared for various relationships, check the following table. 

The Shared cM Project

If you have a specific match to evaluate, enter the number of shared centiMorgans in the box to see the possible relationships and the probabilities for each. If your shared DNA is reported as a percentage, there is an alternate entry box you can use.

Choosing the Right Test for Measuring DNA Relationships 

Autosomal DNA tests can even serve as a cheap but powerful paternity test. Just be aware that tests for children under 18 years of age can only be ordered by a parent or legal guardian. And some genetic genealogy companies have a minimum age requirement for testing.

NOTE: DNA testing from home test kits of any kind is not admissible in a court of law. That’s because there is no way to prove who provided the sample. If you have a legal situation such as child support, custody or immigration, the parent and child need to be tested in person at a certified paternity testing laboratory such as EasyDNA.

Labs like EasyDNA have nationwide collection sites. Test subjects can be in different locations and a chain of custody process ensures valid comparisons. Just be sure to order the LEGAL test and not the Home test.

I did all my testing of close relatives through 23andMe and Family Finder. Each has proven to be excellent for this purpose. This can also be done with AncestryDNA and MyHeritage DNA

The AncestryDNA test lacks a chromosome browser, which may be useful in some cases. Therefore, AncestryDNA is not my first choice for confirming close relationships if that is your only objective.

BEWARE: Many paternity testing companies offer what they call sibling or kinship tests. This is old, outdated technology. They only test a handful of markers and the results are often inconclusive and sometimes wrong. Do not waste your money.

  1. DNA Testing Adviser Home
  2. DNA Basics
  3. DNA Relationships