Genealogy is a very popular hobby but genetic genealogy is slightly different, it is the building of your family tree using primarily the relationships revealed in your DNA. It is a process available to everyone, and the only real avenue for most adoptees searching for their birth parents. Family trees can be wrong, DNA however never lies.
You get approximately half your DNA from each parent. While DNA never lies, there are plenty of variables. A parent-child match is in the 3330-3720 centimorgan range with the average being about 3500cM.
In genetics, a centimorgan (abbreviated cM) is a unit for measuring genetic linkage. It is defined as the distance between chromosome positions (also termed loci or markers) for which the expected average number of intervening chromosomal crossovers in a single generation is 0.01. It is often used to infer distance along a chromosome. However, it is not a true physical distance.
Now that you are completely confused let me assure you, DNA halves with each generation. But of course, all the numbers are approximate as you will see by the chart.
Because of how DNA divides and recombines multiple relationships can be represented by the same approximate number of centimorgans. This is the biggest reason why only DNA or family trees alone will rarely get you there. Each division represents a generation so if you have a match with 900cM it would be back three generations, your parents, your grandparents and then to your great grandparents. This is helpful in quickly determining how far back you share a common ancestor. When I was searching for my father I went back an extra generation to be sure because of the consistently inconsistent number of cMs in DNA splits.
I belong to a number of adoptees groups, looking for an unknown birth parent is very much the same search with the same issues with limited or missing information. So often people get a DNA test and assume their parent will just pop into their matches. Not saying this never happens but it is not the norm. I see people who opine their closest matches are only 1250cM, nothing close enough to identify the missing parent. (I’m 74 and the best I could hope for would have been a half-sibling so I wasn’t expecting a lot of close matches). They routinely discount matches in the 100cM range as worthless. Fact is every match is information you can use. Obviously, you want to use your best matches but sometimes the only matches you have are distant cousins which isn’t the end of the world so many searchers believe.
I started the search for my father with a 23cM match, a 4-6th cousin at best. They had a tree and three generations with names (later generations were still living and marked “Private“). I checked for shared matches and found another cousin at 31cM and another larger tree that miraculously connected with the first. I hadn’t found our common ancestor but I kept building using Census records etc. I was confident two of the family names I needed to find were Fuhrman and Gallmeyer, so I searched my DNA matches for people with those surnames in their background. I found a 38cM match along with more people for my tree. A pattern was emerging. It seemed to be two families all living in the same town in Indiana. They were German immigrants, while that didn’t fit my ethnic breakdown I was still a generation or two from my Dad. Slowly I built the tree until I had four generations starting with the first ones to immigrate in the late 1700s. Then I got a 53cM match with an extensive tree that brought the families together with two others. So I was looking at four families who were now converging into two, exactly what I needed to find my father.
Even If I never got another match I would have found my father it was just a matter of building the families down to my parent’s generation. Think about that, no match larger than 53cM and I could find him. The DNA matches shared a common ancestor but weren’t necessarily on my direct line. Finally, I got three good matches that were 2-4th cousins and we shared a great grandfather (to me). It was the confirmation I needed to know I was on the right line to find my Dad. The three matches ranged from 157cM to 187cM, still not close to 1250cMs.
Along the way in building out all the children and grandchildren and their children through the generations, I found several potential father candidates. I assumed my father had to be born between 1915 and 1921 (when my mother was born), and he had to be in my home town or close by in the late fall early winter of 1943 when I was conceived. One by one the candidates were eliminated and as it turns out the last possible one was my Dad. He was the right age, in the Army Airforce, attended the Army radio school in my home town in the summer and fall of 1943. Both my mother and grandmother worked at the airbase. The first picture I saw of him was a high school Sketch Club group shot and I would have known him anywhere.
Had I ignored the small cM matches and held out for 1200cM or more I would still be waiting because there are no other children and he died in 1975. This was all done on Ancestry and they provide the tools that make it fairly easy. It was a lot of work and took several months, but so glad I didn’t just give up.
There are a great many people searching today who are under 50 and they may need higher matches to fill in all the necessary family members because the last Census released was 1940. But as the Universe takes away one resource it opens another. Obituaries are a bit easier to find, particularly if you have narrowed down the geographical area. City directories are also a good source of employment information and newspapers. Don’t discount Google as a resource, it was a simple Google search of my Dad that gave me his connection to the Trentonian newspaper. While that information didn’t help me find him it did flesh out what I knew about someone I could never meet.
Use the DNA to put and keep you on the right track and a tree with research to fill in the blanks. With DNA virtually anyone can be found with patience.
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