Autosomal DNA testing is a widely used method for confirming biological relationships, and testing a grandparent is a common strategy in genealogical searches. This test examines the DNA inherited from recent ancestors and can definitively verify a biological link between a grandchild and a potential grandparent. Interpreting the results requires moving past simple percentage figures to understand the specific measurements provided by testing companies. The primary goal is to demystify the match interface by explaining the fundamental units of genetic relatedness and how to analyze the shared genetic material.
The Foundation: Understanding Centimorgans
Genetic testing platforms use a specialized unit called the Centimorgan (cM) to measure the amount of shared DNA between two individuals. A Centimorgan is not a physical measure of length; instead, it represents a unit of genetic distance or linkage. One Centimorgan corresponds to a one percent chance that a section of DNA will separate, or recombine, during the formation of reproductive cells over a single generation.
The total number of Centimorgans shared with a match is a more reliable indicator of relatedness than a simple percentage. Inheritance is random; while 50% of DNA is received from each parent, the exact proportion inherited from grandparents varies due to recombination. This variation means that even close relatives, like full first cousins, share different total amounts of DNA. The Centimorgan measurement captures this variation precisely, and a higher total count indicates a closer relationship.
Confirming the Match: Expected DNA Sharing Ranges
When assessing a potential grandparent-grandchild relationship, the total shared DNA amount should fall within a predictable Centimorgan range. For a full biological grandparent and grandchild, the shared DNA is expected to be between approximately 1300 and 2300 cM. This wide range results from the random nature of genetic recombination, meaning a grandchild might inherit slightly more or less than the theoretical average amount from any single grandparent.
The average amount of shared DNA between a grandchild and a full grandparent is around 1700 cM, which translates to a theoretical average of 25% of the grandchild’s total autosomal DNA. Results within the 1300–2300 cM range confirm the close familial connection. Any total outside this established boundary suggests the predicted relationship is incorrect or that a different type of close relative is involved.
The possibility of a half-grandparent relationship must also be considered. This occurs when the grandparent is biologically related to only one of the grandchild’s parents. In this case, the expected shared Centimorgan total would be approximately half of the full relationship range, corresponding to an average of 12.5% shared DNA.
Deeper Analysis: Interpreting Shared DNA Segments
While the total Centimorgan count summarizes relatedness, a deeper analysis involves examining the specific shared DNA segments. These segments are continuous blocks of DNA passed down intact from a common ancestor. Most testing platforms allow users to view these segments on a chromosome browser. A true, close relationship, such as a grandparent-grandchild match, will feature a small number of very long, continuous shared segments.
The length of the longest shared segment is an important secondary metric for confirming a close relationship. Segments below a certain threshold, often 7 cM, are usually dismissed as “genetic noise” or statistical anomalies in genealogy. A legitimate grandparent match will show multiple segments well above this minimum, often with the longest segment measuring over 145 cM. Focusing on the quality and length of these shared blocks helps verify that the DNA match is recent.
Troubleshooting Common Issues and Ambiguous Results
Sometimes, the shared Centimorgan count may fall slightly outside the expected range, or the match may be ambiguous, suggesting multiple possible relationships. This variation can be due to the natural randomness of inheritance or, in some cases, errors in the testing process. If the total is lower than expected, it may indicate a half-relationship.
If a result is unclear, it is helpful to understand the difference between shared DNA that is “Identical By State” (IBS) and “Identical By Descent” (IBD). Identical By State means the DNA sequences match perfectly by chance. Identical By Descent confirms the DNA was inherited from a recent common ancestor. For a close relationship like a grandparent, the shared DNA must be Identical By Descent to confirm the biological link. When results are ambiguous, testing the other parent can help clarify the relationship through parental phasing, which determines which side of the family the DNA segments originated from.