Which DNA test to buy for genealogy?

People ask me often:  Which kind of DNA test to buy?

I tell people they should do an autosomal DNA test.  This is the only test sold by AncestryDNA and 23andMe.  Family Tree DNA also offers this test, called Family Finder.  The price has dropped to around $100 at all three companies.

An autosomal DNA test matches you with other people whose DNA is identical to yours in a few spots, as most will be distant cousins.  These identical areas were passed down from an ancestor common to you and your match.

Diagram of Autosomal DNA inheritance

The goal is to figure out which of your ancestors you have in common.  This is not easy.  The DNA test will not produce a family tree for you.  You still must research your ancestors in records to find the leaves of your family tree.  You may find a close cousin who has done a lot of research and this cousin may provide you with his/her research.  You may find others who help you work past a tail end in your tree suspected of holding the ancestor in common.  If you were adopted, you may find close relatives who can help you identify your biological parents.

If you are serious about finding relatives, you need to test your DNA at all three companies.  You may have a close relative at one site who is not going to test multiple times to find other relatives.  You have to find them by testing at all three sites.

Confusion arises with the Y-DNA and mtDNA tests.  These tests are offered by FamilyTreeDNA and vary in price based on number of tested markers.

One ancestral line only:  Y-DNA testing and mtDNA testing

The Y-DNA test is for males.  Testers are matched to other men who share an almost identical Y chromosome.  The value of this test is that you don't have to figure out which ancestral line holds the common ancestor.  The common ancestor will always be on the direct paternal line, father to son, because this is how the Y chromosome is inherited.  Fewer differences with a match indicate that the common ancestor is within fewer generations.  This test is useful in surname studies, if your direct paternal line comes from a community that perpetuates last names from father to children.

The mtDNA test (mitochondrial, not maternal) is similar to the Y-DNA test in that the common ancestor will be in one ancestral line, mother to daughter.  MtDNA is passed on from mother to children, both sons and daughters.  Men and women may take this test.  A man will not pass his mtDNA on to his children, so that is why the test-taker can be male, but the common ancestor will be found in his mother's direct maternal line.  Unlike the Y-DNA test, the most recent common ancestor could be thousands of years ago, as mtDNA does not vary generation to generation as much as Y-DNA.

These Y-DNA and mtDNA tests provide your haplogroups, which is useful in tracing ancient migratory paths of humans across the globe.  23andMe provides predicted haplogroups with their autosomal test.  This is not the same as taking a Y-DNA or mtDNA test.

Anyone who has been researching their family tree should try the autosomal DNA test.  Your tree will grow.