DNA and Genealogy

In the United States there are three big DNA testing companies: Ancestry DNA (not to be confused with DNA by Ancestry), 23andMe, and Family Tree DNA. Another site that many find useful is Gedmatch. Gedmatch does not do DNA testing but does have some great tools to help you analyze your DNA and find DNA cousins.

Each company has it pluses and minuses. Follow the above links and I will tell you what I personally have done, and then leave you to make up your own minds about what route that you want to go if you decide to do DNA testing. But let us start with some of the basics of DNA is and how we can use it in conjuction with tradition genealogy research.

The scientists measure segments of DNA using Centimorgans (cMs), which by necessity is a very small measurement. The more cMs two people have in common the more closely they are related. A child shares half of their DNA with each parent.

To give some real life examples:

This shows just how randomly we all inherited the DNA from our common ancestors. I should also mention that by the time you reach the 4th cousin level you only have a 50% chance of sharing ANY DNA with that cousin. Just like with any rule there are exceptions to this rule, due to the randomness of how DNA is passed down.

Sometimes a larger segment is passed down intact for many generations which can show up as a match in people who are 4th to 9th cousins. There are also times when you can match someone who has two large segments from two different ancestors so the common ancestor isn't as close as it may appear. This can also happen when the common ancestors were part of small tight knit communities where there was a lot of intermarrying.

There is a really great chart showing the average as well as the range of shared DNA different levels of relatives are likely to have available on The Genetic Genealogist's blog. For those that want to see the ranges that show up in my family here is a chart showing that information:

There are many excellent websites and blogs talking about Genetic Genealogy and I don't want to spend a lot of time repeating what others have already said so I will give a brief overview and then add links to some of the other really good discussions at the bottom of this page for those that want to learn more.

Let's start with some basic biology. We each have 23 pairs of chromosomes. One half of each pair comes from our father and one half from our mother. However, the way we inherit the DNA from each of them is random. We might get more of our DNA from our paternal grandmother than our paternal grandfather, and we might get more DNA from our maternal grandfather than from our maternal grandmother. Our sibling might get the exact opposite. That is why siblings don't look identical (unless of course they are identical twins).

My son shares 3720 cMs with me, and my daughter shares 3719 cMs with me.

My son and daughter share 2839 cMs with each other.

My paternal first cousin and I share 773 cMs.

My son shares between 31.3 cMs and 330 cMs with five different 2nd cousins.

My daughter shares between 114 and 175 cMs with those same five 2nd cousins.

My 3rd cousin once removed and I share 44 cMs.

I belong to a Facebook group which has helped me learn about DNA testing and how to understand the results. The group is: Ancestry-GEDmatch-FTDNA-23andMe-Genealogy-DNA. It is a closed group that you have to ask to join, and only members can see the posts. If you do join the group please be sure to read the rules which are pinned at the top of the group.

Thank you Kitty Munson Cooper for helping make sure the information on this page was accurate and clear.