Deep Into DNA
a monthly column by Mike Maglio
We all have 46 chromosomes, 23 from mom and 23 from dad. Two of those chromosomes are the sexy kind, X and Y. We’ll ignore those for now. The other 44 chromosomes are your autosomes. Autosomal testing has become available only within the last few years and has become a very popular choice. It offers the broadest range of results, including cousin matches, ethnic proportions and health indicators.
The biggest difference between autosomal testing and y-DNA or mitochondrial testing is the scope of the results. Y-DNA and mt-DNA will give you data allowing you go back hundreds of generations, but only along the paternal or maternal lines, a small fraction of your genetic history. Autosomal DNA will shed light on all of your ancestral lines, but only back about 5 to 6 generations. Y-DNA and mt-DNA rarely change from generation to generation. Autosomal DNA changes with every generation.
In a perfect world, two siblings would have 50% of the same DNA and 1st cousins could share 25% of their DNA. Two 2nd cousins might have 1/8, 3rd cousins – 1/16, 4th – 1/32 and 5th cousins – 1/64. A 64th is a little more than 1% shared DNA. Autosomal DNA from mom and dad combine and ‘water down’ the DNA from previous generations. Only a small portion of your DNA comes from a specific gg-grandparent, which makes it hard to match further back than 5 to 6 generations.
The goal in autosomal testing is to find long matching sequences (chunks of DNA). Depending on how long the sequences are and the total number of matching sequences, a calculation predicts the cousin relationship.
The previous diagram shows, in orange, 11 matching sections with the longest section on chromosome 18. Autosomal sequences are measured in centiMorgans. One centiMorgan is about 1 million base pairs of genes. The orange sections represent about 36 cM in total and the largest section is about 15 cM. This translates into a 4th cousin relationship. Each testing company has its own set of calculations for determining relationship.
Typically, your results are only compared to other customers from the company you tested with. Since 23andMe and FamilyTreeDNA both use the same DNA sequencing system, it is possible to export your data and compare with records from the other company. Your exported data can also be imported to a 3rd party site called GEDmatch.com for additional matching capabilities.
Once you have your list of cousin matches, the next step is to compare surnames. If all your newly found cousins have filled out their DNA record completely then they have listed all their known surnames and perhaps geographical locations. Finding a common surname is the best way to connect. The catch is that most people who are getting autosomal testing are doing it because they don’t know many of their surnames.
The biggest reason I had my autosomal DNA tested was that I don’t know who my grandmother’s father is. I don’t know his surname. For adoptee and illegitimacy research, the strategy is to divide your autosomal cousins into two groups. One group in which there are surname matches and another in which there are no matches. The group with no matches can contain the missing biological surnames. Look for a surname that is common across this group. A name that shows up often could represent shared DNA.
The next great thing that autosomal testing can do for you is give you your ethnic ancestry proportions. When this test first came out only a very rough split between African, Asian and European ancestry could be graphed on a pie chart. Today, much more detail is available, mapping to dozens of ethnicities.
If I look at my grandparents, my Italian grandfather connects to the Caucasus (a long story on my y-DNA side), my Irish grandmother connects to the British Isles, my Scottish grandfather also connects to the British Isles and that leaves my German grandmother with the Eastern Europe connection.
Don’t be surprised by the results. Testing may reveal an ethnicity that you didn’t know about. If I hadn’t already learned through y-DNA about my Caucasus ancestry, then the Persian wedge would have had me asking if someone made a mistake. On the other hand, you may know for certain that you have Native American ancestry through a mitochondrial test, but the autosomal test doesn’t show it. Don’t panic, your Native American autosomal DNA could have been ‘watered down’ through the generations to a point where it doesn’t register on a test.
For additional ethnicity information, you can import your results from 23andMe or FamilyTreeDNA into GEDmatch.com. Unfortunately, Ancestry does not have an export capability.
The other important set of results from autosomal testing isn’t for everyone. How healthy are the genes that your parents gave you? Within your autosomes and your X chromosome are health indicators. If you wanted, you could find out how susceptible you are to Alzheimer’s, Parkinson’s, diabetes or hundreds of other health issues. The results show either a reduced or an increased risk factor for each ailment. I have a 1.15 times higher risk than average for migraine headaches. Of course, I already knew that.
23andMe offers health indicators as part of their package. The results from FamilyTreeDNA can be exported to a program called Promethease from snpedia.com. Since you can’t export Ancestry results, you can’t get health indicators from that test.
The testing companies, 23andMe, Ancestry, FamilyTreeDNA and Genographic 2.0 do not all use the same DNA sequencing systems. Results can vary. You could try a test from each. Results can vary from sibling to sibling as well. In the case of two full siblings (not twins), you might think that genetically they are a 100% match. Dad gives each sibling 23 chromosomes, half of his DNA. It will not necessarily be the same 23 chromosomes. Mom does the same.
Let’s look at the two extremes. Imagine mom’s DNA as two chunks of 23 chromosomes each – A & B. Dad has two chunks also – C & D. Mom gives each child chunk A and dad gives each chunk D. Both children will have A & D and will be exact genetic matches. What if mom gave one child A and one child B. Then dad gave one child C and one child D. The full siblings would be A & C and B & D, showing no match at all. The truth is that a full sibling match will exist on a continuum somewhere in between, sharing about 50% of their DNA.
The inheritance of autosomal genes is imperfect for genealogical matching purposes. There is a probability of false negatives. If you receive negative results for two people that you expect to be related, then I recommend that you broaden the scope of your project by testing more siblings or cousins.
There are many great reasons to have your autosomal DNA tested. You’ll find cousins, learn about your ethnicity and benefit from health indicators. Your DNA will be in your testing company’s database. As more people are tested, more cousin connections will be found, more detailed ethnicity information will become available and more accurate health risks will be released. Autosomal testing is one step closer to having all your DNA tested.
© Mike Maglio 2012
Mike Maglio is the author of Deep into DNA, a monthly column in The In-Depth Genealogist which focuses on the use of DNA in genealogical research. Mike can be found blogging at OriginHunters.