This time last year I couldn't have told you very much about chromosomes and now I'm swimming in them. These days I'm focusing my efforts on how various descendants of Thomas and Judah Farrell have inherited certain chromosome segments along ancestral lines. (You can read about the larger project here.)
As we each pursue our various genealogy projects, it often causes us to think deeper about the topic at hand and this sure has been true for me while chasing chromosomes. As regards this project I've stumbled into some conclusions and want to share them. These conclusions are very much based on the way our project is shaping up. Our Farrell project is unique to us but there might be something of interest to others. Maybe.
We know that chromosomes are key to unlocking the science of inheritance. If there's a chromosome match that follows some basic parameters then descent is assured (no matter what the records say or especially what they don't say). But for our project, what are those parameters? Here's our thinking and the parameters we came up with. Just remember that I'm no expert on genetic genealogy:)
1. We need to see the match plus a tree in order to know who the common ancestor is. If there's a match but no tree (or the tree is poorly developed) we've found that we're mostly out of luck, unless we want to invest a lot of time helping the matching person build out their tree. Right now we have only six people who have matched and have identified a common ancestor, but we've just started.
2. Importance of the number of centemorgans, or cMs, in the match: more = better. So we know that cMs is how the match is measured and that more cMs equals a better and stronger match. The general rule of thumb is that under 5 to 7 cMs is probably not worth spending too much time on: glance at it and move on quickly. Over 5 - 7 is called IBD or Identical By Descent to indicate that the matching chromosomes are shared by descent, with some level of confidence. Less than 5 is called IBS or identical by state, meaning that the chromosomes are shared most probably by chance or if by a common ancestor, then that ancestor is way back.
As a side note here, at each generation we inherit only about 50% of our DNA from one parent. Which DNA segments are handed down is close to random. By about 5 to 9 generations you start to lose ancestors entirely leaving no trackable DNA.
3. Endogamous populations are a little different and Magnolia, Virginia, now West Virginia, might be one. It was a small relatively isolated community and there were many intermarriages. An endogamous population is a group or social set in which there's mating amongst a small population and therefore the gene pool is shared and re-shared repeatedly. In this endogamous population situation the rule is to use 10 cMs as a cut off point for IBD and establishing a true genetic connection. This 10 cM cut off could be a good tool in evaluating our Farrell matches.
4. Number of generations to a shared common ancestor. We've seen that when it comes to number of generations to that common ancestor, fewer is better and gives us a better chance at spotting a common ancestor. At the point of 4th cousin it is commonly thought that there's maybe a 50-50% chance of the match being reliable. With each generation the confidence drops some more. We want 4th cousins or closer. (Who doesn't?)
5. In looking at the individual chromosomes where there is a match, the key element is the start and stop locations. If two people have a matching segment of over 10 cMs on the same chromosome with greatly similar start and stop locations, that's a good match for us to pursue.
And what are our "best case" requirements?
* A tree that shows the common ancestor
* 7 cMs minimum and 10 cMs or more is best
* 4th cousin or closer, 5 generations or fewer to common ancestor
* Matches with similar start and stop locations on chromosomes
So there we are. We've found that most matches don't work out because we're thinking that they aren't "good enough." We'll either have to wait for more people to be tested that meet our requirements or invest more of our own time helping matches that follow the four generation and greater than 10 cMs rules build out their trees in an effort to find that most recent common ancestor. We will have to evaluate each match and, using these parameters, decide how much time we can invest to make the match gain more confidence and eventually yield that ever-elusive and critically important Most Recent Common Ancestor.
This is at times difficult work, but so very rewarding.
Resources I like:
Beginner's Guide to Genetic Genealogy
Lots of resources and love the way learning is broken down into manageable lessons. You can pop in and learn on the fly or brush up on a topic. Love it!
Blaine T. Bettinger's Blog: The Genetic Genealogist
It's all good here. Check out the Product Reviews to sort out vendors.
CeCe Moore's Blog: Your Genetic Genealogist
I really don't know how CeCe gets it all done. Her schedule makes me swoon with exhaustion. Use the search box down a little ways on the right to connect with any topic you can think of.
ISOGG Blog: the International Society of Genetic Genealogy
The Source. Check it out.
DNAeXplained – Genetic Genealogy
Roberta Estes's web site is a good read. Subscribe or pop in to browse or search.
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