What is Genetic Genealogy?
Genetic genealogy is the process of attempting to determine family relationships by combining the science of DNA testing with traditional document-based genealogical methods.
DNA has been part of the human vocabulary for a number of years, even though we might have forgotten it is the shortened form of the word deoxyribonucleic acid. We all know what it means instinctively, while a full grasp of the science requires years of study. We inherit our DNA from our parents and the prior generations that came before them. It is vital in making us who we are.
DNA is simultaneously precise and random. This sentence may appear to be at odds with itself, and this article will attempt to resolve this apparent conflict.
Humans have 46 chromosomes, although scientists typically refer to them as 23 pairs of chromosomes. We also have about 3 billion base pairs. You can envision each of the 23 pairs of chromosomes as an extremely long ladder with about 150 million steps (made from the base pairs). The entire ladder is twisted, forming a double helix structure.
The 23 pairs of chromosomes consist of 22 pairs of autosomes and 1 pair of sex chromosomes. Each of our parents contributes half (one chromosome) to each of the 23 pairs.
The sex chromosome, also known as chromosome 23, is different for males and females. Males inherit a Y-chromosome from their father and an X-chromosome from their mother, while females inherit an X-chromosome from each parent. Many people often express this with the shorthand version of XX=female and XY=male.
Therefore, there are three genealogical DNA types in humans and unique DNA tests for each type:
- Autosomal DNA (atDNA) Test: This is the most common form of genealogical testing performed today. It tests the 22 autosomal pairs, making it the best tool for finding DNA matches (genetic relatives) on both sides of the family. The mixing, or watering down, of DNA by each generation limits this type of testing to about five generations.
- Y-Chromosome DNA (Y-DNA) Test: The Y-chromosome passes from father to son virtually unchanged. It does not mix with anything else like autosomal DNA. Although there are slight mutations every few generations, the Y-chromosome remains nearly identical for thousands of years. This type of test is excellent for deep surname testing.
- Mitochondrial DNA (mtDNA) Test: Mitochondrial DNA passes from mother to daughter virtually unchanged, similar to Y-DNA for males. Mothers also pass mtDNA to their sons, but those sons can not pass it on. This type of test is presently the least used of the three due to the human convention of using male instead of female surnames. They can be useful in helping solve some relationship mysteries involving ancestors with multiple spouses.
DNA testing has become an exact science. With the exception of identical twins, every person that ever walked the planet has a unique set of easily testable DNA. You’ve probably seen television crime dramas where the scientist testifies that “only 1 in 700 billion people” could have that DNA. The prosecutor then reminds the jury there are only 7 billion people on the planet. Yes, we are all unique.
There are many DNA testing and analysis websites to help you understand your DNA and research your family tree.
Now It Gets Random
We get half (50%) of our DNA from our father and half (50%) from our mother. Since they both got 50% from each of their parents, we might assume that we get one-fourth (25%) of our DNA from each of our four grandparents. However, that assumption would be incorrect.
Although each generation passes 50% of their DNA to the next generation, the portion they pass is quite random. If this randomness were not the case, then we would all look like twins of our siblings.
The following chart and description can help illustrate this.
Top Row — Great Grandparents
The top row consists of eight people, representing the eight great-grandparents of the person at the bottom of the chart. The chart illustrates each person with a unique color, starting with the four blue figures representing the paternal great-grandfather. The first stick is solid blue, representing the 22 autosomal chromosomes from his father. The second stick is blue with stripes, representing the 22 autosomal chromosomes from his mother. The third stick is shorter and is again solid. It is the Y-chromosome from his father. The fourth figure for this person is the blue circle, and it represents the mtDNA from his mother.
The next group of figures, immediately to the right in yellow, represents his spouse. Since she is female, there is not a Y-chromosome shown. Again, the solid stick represents DNA from her father, the striped stick is DNA from her mother, and the yellow circle is the mtDNA from her mother.
There are six more (three sets) great-grandparents across the top row, represented by different colors. The chart shows the mtDNA circles for the four males crossed out because it cannot be transmitted any further. Males receive their mother’s mtDNA but do not pass it on to any children. Although the eight color groups represent the eight great-grandparents, the solid and striped version of each color indicates there are actually 16 unique DNA contributors (the 16 great-great-grandparents) in this example.
Second Row — Grandparents
Now, we move down to the second row, showing the four grandparents. The first group of figures is the paternal grandfather. The first blue stick is the autosomal DNA he got from his father, and it is paired with the yellow stick from his mother. On closer inspection, we see that about 75% of the first stick is solid blue, indicating it is from his father’s father, and the other 25% is striped, indicating it is from his father’s mother. The yellow chromosome stick from his mother looks to be about 80% solid (from his mother’s father) and only 20% striped (from his mother’s mother).
Therefore, instead of receiving equal amounts of DNA from each of his four grandparents, he got about 38% from his paternal grandfather, 12% from his paternal grandmother, 40% from his maternal grandfather, and 10% from his maternal grandmother. Additionally, he received a clean (unmixed) Y-chromosome from his father and unmixed mtDNA from his mother. The mtDNA is again shown as crossed out because he cannot pass it on.
The other three grandparents in the second row also show random mixtures instead of equal 25% mixes from each of their grandparents.
Third Row — Parents
This process repeats again, creating the third row representing the next generation. Here you can see that the person labeled “Father” has eight unique DNA contributors (four colors in both solid and striped formats). He got most of his DNA from his paternal grandmother (solid yellow) and his maternal grandfather (striped green). His Y-chromosome is identical to that of his father and his father’s father. His mtDNA in pink is identical to that of his mother and his mother’s mother.
Also on this row is the mother, and in this particular example, most of her DNA is shown as coming from her paternal grandmother (striped orange). Her mtDNA is identical to the mtDNA of her mother and her mother’s mother.
Bottom Row — Son
The random mixing of the autosomal DNA occurs once again in the fourth row for the person labeled “Son.” He received 50% of his autosomal DNA from his father, and the first stick consists of a random mixture of his father’s ancestors. The second stick is the 50% of his autosomal DNA from his mother and her ancestors. Of the 16 unique contributors (his 16 great-great-grandparents), the orange-striped DNA occupies the largest space in this final mixture. Some of the other colors are barely detectable, indicating the randomness of autosomal DNA transmission from generation to generation.
However, this final person’s Y-chromosome is still solid blue, passing unchanged from male to male in each generation. His mtDNA also arrived unaltered, passed down from female to female to him. This is visible on the right side of the chart. The Y-DNA and mtDNA from his other six great-grandparents were eliminated in the intervening generations. He can have his mtDNA tested, but it is the end of the line since he is male and cannot pass it on. If he had a female sibling, the opposite would happen. She would pass on the mtDNA but would not have any Y-DNA to pass to her children. In addition, her mixture of autosomal DNA would be different from his.
CentiMorgans (cM)
A centiMorgan, abbreviated cM, is a unit used to measure genetic linkage and equals about one million base pairs. Each chromosome has a different length, and therefore each contains different amounts of genetic information. The cM values can range from about 60 to 280 for each chromosome, and total up to about 3500 cM. It is named after Thomas Hunt Morgan, an American geneticist. The International Society of Genetic Genealogy (ISOGG) offers a more technical definition of centiMorgan.
The following chart is from the Shared cM Tool at DNA Painter. Each box shows the relationship, the average cM of that relationship, and the possible range of values (99th percentile). For example, looking at the “Parent” box, we see an average of 3485 cM and a range of 2376-3720 cM. The “Grandparent” box has an average of 1754 cM and a range of 984-2462 cM.
On average, we have 3485 cM of shared DNA with parents, 1754 cM with grandparents, and 887 cM with great-grandparents. These are only averages, and on average, you can see the amount of shared DNA is cut in half with each succeeding generation.
As illustrated in the mixture example above, these numbers can vary widely for each individual. Shared DNA with a grandparent can range from 984 to 2462 cM. Said another way, a grandparent may contribute as little as 14% to as much as 35% of our DNA instead of the 25% “average.” Shared DNA with a great-grandparent can range from 485 cM (7%) to 1486 cM (21%).
According to the chart, a full sibling can range between 1613 and 3488 cM of shared DNA, clearly showing that although you each got half of your DNA from the same two people, the halves you each got were much different. The “1C” block is for first cousins, “2C” for second cousins, and “2C1R” means second cousins once removed. You can see the cM values drop and the range expands as the relationships get weaker.
When you get to the “3C” block (third cousins) the average is 73 cM, but the range is zero to 234 cM. That is correct, it is entirely possible to have no easily testable shared DNA with a third cousin. This chart and tool are very handy for helping determine where your DNA matches might fit in your tree. Another key takeaway is that every possible relationship has a range.
Many researchers believe that values below 20 cM start to become unreliable. Testing companies typically have their own criteria for establishing a minimum threshold. For example, Ancestry requires that you have at least 8 cM of shared DNA with another tester to be labeled a match. Additionally, its “shared matches” feature requires that you have at least 20 cM of shared DNA with the person that is common to your matches and another tester’s matches.
Most DNA testing websites will typically group your matches by predicted relationships. There are no guarantees for these grouping predictions. You may find a fourth cousin in the second cousin grouping, and so forth. This is not the fault of the testing companies. It is merely the by-product of the randomness of DNA transmission described above.
Additional Resources
International Society of Genetic Genealogy: A resource of additional resources with a free genetic genealogy encyclopedia containing hundreds of articles.
DNA Painter: Tools to help interpret autosomal DNA test results
—
If you are a Rowland or have a Rowland in your family tree, then you need to subscribe to the Rowland Genealogy Newsletter.