The Autosomal DNA Test (offered by all three companies) examines autosomal DNA, also called atDNA, which is the DNA found in all chromosomes in the nucleus of all cells except the single pair of sex chromosomes. (the X and Y chromosomes). These 22 pairs of chromosomes are called autosomes.

 

The autosomes are inherited approximately equally from both parents, one chromosome from each parent. This means that about 50% is inherited from each parent, about 25% is inherited from each grandparent, about 12.5% from each great grandparent and so on. It follows that siblings share about 50%, first cousins about 12.5%, 2nd cousins about 3.1%, 3rd cousins about 0.8% and so on. There are reference tables available which show these percentages and the expected variations which increase significantly as the closeness of the relationship decreases. For example the observed range of DNA shared between 3rd cousins is 0.3% - 2.0%.

 

The atDNA test which cost approximately $100 (US) or less involves testing of approximately 700,000 SNPs on all 22 pairs of autosomal chromosomes. The testing company compares your results with the results of other people who have taken the test and provides you with a list of matches and their estimated relationship and a means to contact them. It also provides an ability to show you the family tree of the match, but only if the person provides it.

 

It will uncover relatives from both branches of your family and companies provide you with a list of those closely matching you from those who have tested with their company with an estimate of the relationship (immediate family, first cousin, second cousin. It cannot tell you directly which side of your family and exactly how that person is related to you. It estimates and in some cases quite accurately the relationship between you and a match by comparing the number of shared DNA segments and the percent DNA shared.

 

Autosomal DNA has rapidly become one of the most commercially successful tools offered to build upon genealogical records obtained solely from documents. But there remains much confusion among people considering taking a test and what to do with the results once you have them. Here are some guidelines that may be useful:

 

Who can take the atDNA test?

Anyone can take an autosomal test, male or female, young or old. The results from older generations in your family (parents, grandparents) who are tested can clarify which side of your family your matches are on. Surprisingly it is advantageous to have a full sibling take the test with you since each of you have half of your parents DNA but not necessarily the same halves. So not all of your matches will be the same as your sister or brother! For example, I have 1,246 autosomal matches on Ancestry.com but my brother has 1,413.

 

What are its strengths?

The autosomal test may be used with equal success by both men and women can find relatives on any of your family lines if they have tested with the same vendor of services. The autosomal test will find most genetic relatives in the most recent genealogical time frame who share the greatest percentages of DNA.  It is recommended for genealogy enthusiasts who are willing to work with potential matches and share genealogical family trees. Considering the price and the detail involved in the testing it is a good value. Discounts are frequently offered.

 

What are its weaknesses?

The weaknesses include determining which branch of your family tree you share with your cousin, the small percentage of people who have already tested with any of the services and the relatively large number of people who don't know or are not willing to share family trees. Greatest success in genealogy like any field of human endeavor comes to those who are interested in collaboration with others. Another weakness of autosomal testing is, like all the tests, it is over promoted by vendors who profit from it and under supported.

 

Recommendations

If you take genealogy seriously take the test from ancestry.com which according to the ISOGG wiki has 2.5 times as many people in their database as their nearest competitor. This is in line with my own findings for the most successful matches and those people who are willing to share family trees.

 

Interested readers can further their knowledge at The ISOGG's page on Autosomal DNA and in the web sites linked above in this section.