Is That DNA Match a 1st Cousin, Half-Uncle/Aunt, or Half-Nephew/Niece?

You receive your DNA results and see that there are predicted relationships for each match. The relationship algorithm takes the number of centimorgans (a measurement for the amount of shared DNA) you have in common with a match and gives a suggested relationship based on the percentage of people who have this same number of centimorgans (cM) in common. Generally, the larger the cM in common, the closer the relationship. However, there is always a range of possible relationships. In the chart above, you can see that someone who has 902 cM in common with you might be your 1st cousin, your half-uncle or aunt, or your half-nephew or niece.

Another measurement used is DNA segments that are shared – the number of segments and the length of each segment. Sharing fewer, longer segments often indicates a closer relationship.  However, this also cannot pinpoint the exact relationship. Take a look at the examples below:

The cMs in common predict the possible relationships to be uncle or aunt, niece or nephew, grandparent, or grandchild. The cM in common suggest the niece is a closer relationship to our source. The fewer, longer segments for the aunt, however, suggest she is a closer match to our DNA source. However, an aunt is not a closer or more distant relationship than a niece.

In this chart, based on the cMs in common with our source, the first row shows possible relationships of 1st cousin, half-uncle/aunt, half-niece/nephew, great-grandparent/grandchild, or grandniece/nephew. The second row shows these same possible relationships with the addition of 1st cousin once removed (1C1R) or half-1st cousin. The first row shows a greater number of total cMs and segments and a longer segment than the second row, yet this doesn’t determine that the match is any closer in relationship to our DNA source. Both are known 1st cousins from different siblings of the DNA source’s parent; the centimorgan and segment measurements do not differentiate between the predicted relationships and one 1st cousin is not any closer or more distant than another 1st cousin.

So how can we determine which of the range of predicted relationships is the correct one? The first and most obvious way is to look at dates of birth. If a match was born a generation (20-30 years) before the source, that would generally lead us to the older relationship (grandparent instead of grandchild.) The next thing we look at are official documents: birth certificates, death certificates, marriage certificates, census records, etc. These can confirm parents’ names, dates, ages, siblings. Using these documents, we may be able to further narrow down the relationship. Sometimes, however, there aren’t records to be found or there is missing information, such as a parent’s name, on a vital document. In these cases, we can only speculate as to the actual relationship, and will have to rely on corroborated family stories, critical thinking, and logic to make up our own minds.