Decoding DNA Relationships: Understanding the Connection Between Adoptees and Their DNA Matches

Q. I am 33-year-old adoptee who has tested their autosomal DNA at 23andme.com, AncestryDNA and Family Tree DNA. AncestryDNA suggests my closest DNA match as “Close Family–First Cousins.” We share 1,622 centimorgans of DNA across 75 DNA segments – this person does not have a family tree attached and has not uploaded the results to GEDmatch; furthermore I don’t know their age – what could our likely relationship be?
A. Adoptees who pursue genetic genealogy testing hope to identify a close match, as this can reveal their genetic heritage. Once they do so, the next step should be identifying their most likely ancestor’s genealogy in order to better understand where this person fits within your family tree that’s beginning to emerge.

All testing companies now provide information on the total amount of DNA (measured in centimorgans, or cMs) shared between genetic matches – something which can help establish genealogical relationships. A centimorgan measures distance between genetic markers on DNA based on expected frequency of recombination between generations; generally speaking one cM equals one million base pairs but this can vary considerably.

Once you know the number of shared centimorgans, your first stop for estimating a relationship should be the International Society of Genetic Genealogy’s (ISOGG) Autosomal DNA Statistics page. Here you’ll find a table which sums up average amounts of DNA (in both percentages and centimorgans) shared across many genealogical relationships – see page opposite for reference.

Remind yourself that these figures represent only an average; thus, actual shared DNA amounts will likely differ depending on which relative it comes from; often more or less than expected will be shared, especially if there are more recent common ancestors between two relatives.

AncestryDNA reports that you and your close match share 1,622 cM of DNA; with estimates for such relationships at 25% shared between actual and estimated DNA levels – which makes for high confidence that one or more of these relationships exist between you. As long as no multiple ancestors exist between you, any relationship you share would fall under one or more of these categories.

Shared cM Project can also be an invaluable resource, collecting data on actual ranges of shared cM for different genealogical relationships. Last year alone, this resource collected more than 7,500 data points pertaining to amounts of shared DNA between various genealogical relationships as well as visualizing distributions for ranges shared among relationships.

“Degree 2” relationships encompass Aunt/Uncle, Grandparent/Grandchild and Half Sibling relationships. A shared amount of 1,622 cM falls within this relationship type but not any others; for instance if 600cm was shared it could fall under Degree 5/6 range and provide for even more potential relationships to exist between these types.

With ISOGG and Shared cM Project pages as your guides, chances are high that one of these male AncestryDNA matches are your close match:

At age 33, you’re unlikely to be grandparenting yourself) your grandfather (unless a full sibling has died). Or it could be one of their sons or daughters; then again it could also be another half sibling who has left him a sibling behind, in which case both sets of family would share more DNA (in this instance approximately 850cM would be shared between you two).
Age may help determine whether someone is your half-brother; however, if that information cannot be obtained directly, AncestryDNA offers a tool called Shared Matches which you can use to investigate further this relationship.

Shared Matches allows you to identify matches who appear both on your and a relative’s match lists. Though you can search for shared matches with anyone from your match list, only individuals you share at a fourth cousin level or closer will appear here.

If you share any matches with a relative, look at their family tree to gain any additional insight into this individual – possibly your grandfather, uncle, nephew, or half-brother.