Low Centimorgan Matches (cM < 20) find a purpose in research.
An update to The California Nugget Spring 2024 article.
by Stewart Blandón Traiman, MD
[This article was originally published in The California Nugget, Spring 2025 edition.]
Introduction
In an article for the Spring 2024 issue of The California Nugget I examined the analysis of autosomal DNA matches on Ancestry in an effort to identify the mystery father of Ernest Piatt (1906-1940). Ernest was one of four children born to a single mother near the oil fields of Monroe County, Ohio. Research narrowed the DNA matches to a twelve-cousin cluster, all sharing genetic links to the Piatt-Grant lineage. The likely father was one of four sons of Robert Sloan Grant and Nancy (Cousins) Grant: James, Marion, Ralph, or Charles. However, the available evidence was insufficient to definitively determine which brother was the biological father.
In late 2023, Ancestry introduced Pro Tools, available through an additional monthly subscription. The tools include Tree Checker, Insights and Reports, Smart Filters, and Enhanced DNA Tools. The last feature is a game-changer for tracing DNA relatives and triangulating connections. Previously, Ancestry-DNA(R) matches displayed only the tester’s shared centimorgan (cM) amount and potential relationship to each DNA cousin. This new tool extends that capability by providing the same data for the shared matches’ matches. This additional layer of information makes it possible to determine how DNA cousins relate to one another, not just to the tester. Can Ancestry’s new Advanced DNA Tool provide new evidence to solve the mystery of Ernest Piatt’s father? I found several positive results that strengthened DNA knowledge and connections among the twelve cousins in the Piatt-Grant cluster. By uncovering previously unknown DNA matches on the Grant maternal side, the results further support the hypothesis that Ernest’s father was one of the four Grant sons.
How It Works
The first step was to examine each of the twelve cousins’ “Shared Matches” lists and review the amount of centimorgans (cM) they shared with their DNA matches. The new tool now displays three columns (figure 1). The first two columns are familiar to any user of Ancestry DNA–they display how the main tester relates to the cousin’s matches. The new third column is where things get interesting, as it displays the cM and presumed relationship between the cousin and each of their matches. This new layer of data triangulates how the tester and cousin relate to the same person.
When viewing a cousin’s match list, it is essential to sort by the third column. The match list will display in descending order by cM quantity, bringing the highest matches to the top of the third column. This method quickly revealed several significant findings.
The anonymized cousin coding system from the previous article was reused to maintain privacy and consistency when discussing findings. The main tester, a third-great-grandson of the mystery man, was coded as J.T.
Immediate Results
Several high-quantity centimorgan matches were immediately found by using the sort function. The shared centimorgans are high enough to prove the relationships with 100% accuracy as verified by The Shared cM Project.1
- Cousin 64 shares 2,546 cM with his sister that was previously unknown. She’s been added to the genogram (see figure 2 on page 16 for full genogram) as Cousin 64A. She shares 9 cM with the main tester, J.T.
- Similarly, Cousin 150 shares 2,531 cM with a newly revealed brother. He shares 10 cM with J.T. and has been labeled Cousin 150A on the genogram. According to The Shared cM Project 4.0 tool v4, both of these shared cM values are consistent with full siblings.
- Previous research identified that Cousin 403 had one child, Cousin 405. However, Pro Tools revealed a second child, now designated as Cousin 405A. Cousin 403 shares 3,450 cM with Cousin 405 and 3,460 cM with Cousin 405A, confirming a parent-child relationship with both. Additionally, Cousin 405 and Cousin 405A share 1,429 cM, consistent with a half-sibling relationship. The newly discovered Cousin 405A shares 10 cM with the main tester.
- Cousin 1777 has three newly identified relatives with significant amounts of shared DNA. He shares 1,998 cM with a half-brother or nephew, who also shares 9 cM with the main tester. Another half-brother or nephew shares 1,663 cM with Cousin 1777 and 11 cM with J.T. Additionally, a grandniece or half-niece shares 978 cM with Cousin 1777 and 10 cM with J.T. These individuals have been coded as Cousins 1777A, 1777B, and 1777C, respectively.
- A review of the three new cousins’ shared match list reveals that 1777A and 1777B are brothers, sharing 2,440 cM. Furthermore, 1777A is the father of 1777C, as confirmed by their 3,466 cM shared DNA. These relationships were also consistent with ThruLines® data.
This table presents shared centimorgan values between the twelve cousins in the Piatt-Grant cluster and the six new additions. The cMs shared with the main tester are in the second column. The cousin codes are shown in the rows and columns, and the intersecting cells show the amount of shared DNA between them. Cousin 8 was the anchor for this research cluster.
Notably, the six newly identified relatives share low centimorgan (cM) quantities with the main tester, all below 11 cM. Before Pro Tools, Ancestry did not display matches sharing less than 20 cM, as this cutoff was a practical limitation; very low shared DNA makes it extremely difficult to place an individual on a family tree. Additionally, with thousands of low-cM matches, it was more effective to prioritize the highest matches first. These six new cousins were not discovered using pre-Pro Tools methods.
With the Pro Tools Enhanced DNA function, low-centimorgan matches (LCMs) now gain purpose and value in research. The 20-cM cutoff threshold is now applied to the shared matchs’ list rather than to the test-taker’s. This shift benefits researchers by allowing cousins’ high-cM matches to surface LCMs for the tester, which were previously lost in a vast sea of data. The number of matches for each of the twelve cousins increased significantly compared to previous research. While some of this growth may be attributed to new testers joining the Ancestry database, it became evident that the primary reason was the increased visibility of LCMs provided by the new tool. Documenting the newly expanded shared match lists of the original twelve cousins, a new list of 157 additional cousins was generated. On average, these new matches share 13.73 cM with the main tester, indicating that all were LCMs that had been previously excluded by Ancestry’s match threshold. Pro Tools also revealed an error in previous research. Earlier findings had suggested that Cousin 138 was the nephew of Cousin 329. However, Pro Tools shows that these two individuals share 3,481 cM, consistent with a parent-child relationship rather than a pibling-nibling connection.2 The mistake originated from misinterpreting the relationships documented in the obituary of Maryanne E. Grant, who was the mother of Cousin 329 and the grandmother of Cousin 138.3 This relationship has been corrected on the genogram.
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