How Much DNA Did I Inherit from Grandma?

Over the past several weeks, I have written about the three mothers of my grandmother, Fern Laurine Stoddard Woodbury (1929-2020): Opal Gay Coumerilh, Martha Jane Rickman, and Maude Dee Porter. In the midst of working on these articles, Fern passed away on Tuesday 5 May 2020 at the age of 91. I will miss her, but I also find peace in her passing. Fern’s husband, Frank, died in 2001. In the later years of her life, Grandma would often comment how much she missed Frank and how much she longed to be with him again. After a fall and injury a few years ago, her health gradually declined, and the last year was particularly difficult for her. The pain and suffering of the last few years, stand in contrast to the peace and joy that I hope and believe she now feels upon being reunited with her husband.

Grandma and Grandpa Woodbury were the ones to initially spark my interest in family history and genealogy with the present of a Family History binder. When my interest grew into a career as a genetic genealogist, Grandma would often let me know that she was “well pleased” with the work that I was doing for our own family and for others. We often had long conversations about the discoveries I was making about her family and their history.  

I am fortunate that Grandma agreed to perform DNA testing at my request several years ago. At other times, I have written on the importance of inviting your older relatives to perform DNA testing. When an older relative dies, the DNA that they carried and the genetic record they inherited from their ancestors is often lost forever. While testing their descendants can help to piece together elements of their genetic profile, this process requires multiple tests for several children or grandchildren and even then, often only results in partial reconstruction of their DNA.

When I asked Grandma to take a DNA test, we talked over what the test results could reveal, discussed how I would be using the test, and arranged for me to administer her test accounts. I helped her take two DNA tests: one at Ancestry.com and one at Family Tree DNA. We transferred the test results to GEDmatch. Once the tests completed processing, we reviewed the results together, talked through the ethnicity estimates and explored the genetic cousin match lists. As I made additional discoveries, I kept her updated on how her DNA test results were helping us solve family history mysteries. Thus, DNA testing and genetic genealogy became a shared and treasured experience for both of us.

Each of us inherit exactly 50% of our autosomal DNA from our parents. Beyond that the amount of DNA we inherit from any given ancestor is only approximate because of recombination: a random reshuffling of DNA before it is passed on to the next generation. We get about 25% from each grandparent, about 12.5% from each great grandparent and about half again for every more distant generation of ancestors. This process of recombination is also the reason why we share chunks or “segments” of DNA with our genetic cousins. Three other types of DNA (Y-DNA, X-DNA and mtDNA) follow different inheritance patterns. Y-DNA is inherited along the direct paternal line. X-DNA can be inherited from a subset of particular ancestors. For males, X-DNA is maternally inherited. For females, meanwhile, X-DNA is inherited from their mother and from their paternal grandmother. Mitochondrial DNA is inherited along the direct maternal line of ancestry. In the case of Grandma Woodbury, I only inherited autosomal DNA from her. She was the mother of my father, so I did not get any Y-DNA, X-DNA or mitochondrial DNA from her.

So, how much DNA did I get from Fern? Though the answer to that question may not help me genealogically, I still find it interesting to discover which of my grandparents contributed the most to my DNA, and you might too. According to Ancestry.com, Fern and I share 2009 centimorgans (cM) of DNA with each other on 48 segments.[1] Centimorgans are a unit of measurement to communicate the likelihood of recombination between two points on a chromosome. Consideration of total shared centimorgans can help in estimating the level of a relationship. According to Family Tree DNA, Grandma and I share 1920 cM on 23 segments of DNA.[2] At GEDmatch.com, Grandma and I are reported to share 2059 cM on 23 segments.[3] Why is there such a significant difference between all three companies? We will talk about that in a future post!

Though there is some variation in the reported amount of DNA I share with Fern at the different companies, each result leads to essentially the same conclusion: I share an appropriate amount of DNA with Fern given our proposed relationship as a grandmother and grandson. By taking the amounts of shared DNA from each company and comparing those values against the reported ranges in the Shared cM Project, I find that all of these amounts of shared DNA are well within the range expected for a grandparent-grandchild relationship which typically fall between 984 and 2462 cM. By plugging these same numbers into the Shared cM calculator at DNAPainter, I also find that depending on the value used, the probability that I am a grandchild of Fern ranges from 91-100%.[4] All other possible relationship levels can easily be ruled out with genealogical context. The DNAPainter calculator also gives us an approximation of the percentage of DNA that I inherited from Grandma Fern: Anywhere from 25.7 to 27.6% of my DNA. However, because centimorgans do not follow a linear scale, and because centimorgan values depend on the position of a DNA segment on a chromosome, approximations of percentages of shared DNA from centimorgan counts are not exact. This is particularly true when you are dealing with very large segments of DNA.

To better estimate the percentage of DNA I inherited from Grandma, I turn to GEDmatch and Family Tree DNA which report the segments of DNA I share with her and the start and stop positions of each shared segment. First, I subtract each start position from the corresponding stop position for each segment. Then, I add all of the resulting values together to determine how many total basepairs I share with Grandma. Finally, I take that sum and divide it by the total number of basepairs in my genome. On this last step, it is important to determine which build of the human genome is being utilized.

Since the initial release of the human genome reference sequence in 2003, several versions or “builds” have been released to close gaps in the sequence and correct previous versions. The most recent build from December 2013 (with updates as recent as December 2019) is Build 38. Family Tree DNA’s chromosome browser currently reports segment boundaries using build 36 of the human genome. GEDmatch allows comparisons in builds 36, 37 or 38. Information on these genome builds can be found through the National Center for Biotechnology Information (NCBI).[5] Though each build reports a total genome size, the total number of basepairs in your genome will depend on your biological sex. Females have two sets of autosomal chromosomes (1-22), two X-chromosomes and mitochondrial DNA. Males have two sets of autosomal chromosomes (1-22), one X-chromosome, one Y-chromosome and mitochondrial DNA. Because the Y-chromosome is smaller than the X-chromosome, males actually have about 98 million fewer total basepairs in their genome than females. Also, because some regions of your DNA have variable numbers of tandem repeats, not every person has the exact same number of basepairs in their genome. Chromosomal abnormalities and mutations can also affect the total number of basepairs in an individual’s genome. However, by consulting the data for a build and adding the total lengths of each chromosome you carry (given your biological sex) you should be able to approximate your genome size in any given build of the human genome. If you are hoping to calculate the percentage of DNA you inherit from an ancestor using this same approach, consult the following table for the total number of basepairs reported in builds 36, 37 and 38 of the human genome for males and females.

Chromosome	Build 36	Build 37	Build 38
1	248,462,984	249,904,550	249,698,942
2	243,136,720	243,199,373	242,508,799
3	200,101,083	198,022,430	198,450,956
4	191,915,711	191,535,534	190,424,264
5	181,001,553	180,915,260	181,630,948
6	172,725,554	171,115,067	170,805,979
7	159,271,083	159,321,559	159,345,973
8	146,868,636	146,440,111	145,138,636
9	141,119,686	141,696,573	138,688,728
10	135,488,012	135,534,747	133,797,422
11	134,567,678	135,046,619	135,186,938
12	132,349,534	133,851,895	133,275,309
13	114,329,838	115,169,878	114,364,328
14	106,368,585	107,349,540	108,136,338
15	100,923,261	102,531,392	102,439,437
16	88,932,739	90,354,753	92,211,104
17	80,592,355	81,529,607	83,836,422
18	76,121,415	78,081,510	80,373,285
19	64,063,509	59,380,841	58,617,616
20	62,435,964	63,025,520	64,444,167
21	48,224,016	48,157,577	46,709,983
22	49,898,750	51,304,566	51,857,516
X	156,432,922	155,270,560	156,040,895
Y	57,772,954	59,373,566	57,264,655
unplaced		3,675,142	4,485,509
mtDNA	16,569	16,569	16,569
Total (-mtDNA)	3,093,104,542	3,101,788,170	3,099,734,149
Total (+mtDNA)	3,093,121,111	3,101,804,739	3,099,750,718
Total Basepairs Female	6070679745	6077495493	6075984539
Total Basepairs Male	5972019777	5981598499	5977208299

Using the approach described above, I calculated the following percentages of DNA I inherited from Fern Stoddard.

Comparison	Percentage
Family Tree DNA (Build 36)	27.41355%
GEDmatch (Build 36)	27.4066319%
GEDMatch (Build 37)	27.4056064%
GEDMatch (Build 38)	27.3325651%

Despite the variation in my reported total shared centimorgans with Fern at the various testing companies, we see here that the percentage of DNA I inherited from Fern is consistently around 27.3-27.4% regardless of the comparison used for the calculation. In future blog posts, I will share how to calculate the percentage of DNA I inherited from my other grandparents, how to determine percentages of DNA inherited from a grandparent when you have no tested grandparents, and other interesting information you can glean by mapping the origins of your DNA.

For now, I will close by reporting that of all my grandparents, I inherited the most from Fern Laurine Stoddard. Maybe that’s why we got along so well, and maybe that’s why most of my family history research interests have centered around her family. I will miss Fern and her sweet, loving influence in my life. Though she has passed from this life, I am grateful for the legacy (genetic, genealogical, and otherwise) that she has left for me. Till We Meet Again.  

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[1] “Paul Woodbury’s DNA Matches,” estimated close family with Fern Woodbury, sharing 2009 cM across 48 segments, https://ancestry.com, private database, accessed May 2020.

[2] “Family Finder – Matches,” kit 282627 (Paul Woodbury), estimated Half Sister, Grandmother/ Granddaughter, Aunt, Niece with Fern Stoddard sharing 1940 cM (1920 revised), https://familytreedna.com, private database, accessed May 2020.

[3] GEDmatch, “Autosomal One-to-one Comparison – V1.0,” comparison between Fern Woodbury and Paul Woodbury,” 2058.7 cM across 23 segments, https://www.gedmatch.com/, accessed May 2020.

[4]“The Shared cM Project 4.0 tool v4,” https://dnapainter.com, accessed May 2020.

[5]International Human Genome Sequencing Consortium, “NCBI36,” https://www.ncbi.nlm.nih.gov/assembly/GCF_000001405.12/#/def, accessed May 2020; and,

International Human Genome Sequencing Consortium, “GRCh37,” https://www.ncbi.nlm.nih.gov/assembly/GCF_000001405.13/#/def, accessed May 2020; and,

International Human Genome Sequencing Consortium, “GRCh38,” https://www.ncbi.nlm.nih.gov/assembly/GCF_000001405.26/#/st, accessed May 2020.