Genealogists typically use three types of Direct-to-Consumer DNA tests: autosomal, Y-DNA, and mtDNA. Some companies also offer tests designed to tell you about your health, but these are not designed for answering genealogical questions.
Autosomal DNA is from the 22 chromosomes that are non-sex-related you received from both parents (about 50% from each). The autosomal DNA test is designed to find living relatives on any of your ancestral lines within about the last 5 generations. If you are trying to confirm a relationship with someone else who is a 3rd cousin or closer, the autosomal DNA test is recommended. Please note that this test cannot in all cases distinguish between matches from your mother's side versus your father's side, especially if both sides of your family are Ashkenazi. Some testing companies use “phasing" to divide your matches by parent, but this is not 100% reliable. Autosomal DNA tests also provide you with a breakdown of your ethnic makeup by percentage, which is known as your “admixture.” Both men and women can be tested.
Y-DNA refers to the DNA found on the Y chromosome, which is only passed from fathers to sons without recombining. Your Y-DNA results can tell you who you share your paternal line ancestor with in recent generations and also about your paternal ancestors' likely migration patterns going back thousands of years.
Of the major DTC testing companies, only FamilyTreeDNA offers Y-DNA testing, in the form of two different types of tests: STR tests and the Next Generation Sequencing test.
FamilyTreeDNA’s STR tests compare your results at a small number of locations against the results of everyone else in their database at those same locations. You can test 37, 67, or 111 markers (locations). This provides you with a match list of other people with Y chromosomes who share a paternal line ancestor within the past 1,000 or so years. This is considered an entry-level test and gives you a list of matches, but can include coincidental matches and there can also be missing matches because the STR markers are inherently more prone to random mutation. This is a low-resolution type of test. You will be assigned a haplogroup (the general branch of the human Y chromosome family tree), but it will usually be an ancient haplogroup (3,000-10,000 years old) and not very useful for understanding where your paternal line ancestors came from or their migration routes.
FamilyTreeDNA’s Next Generation Sequencing test is called the Big Y 700 test. In addition to the STR tests mentioned above, the test scans tens of millions of locations on the Y chromosome looking for single mutations (Single Nucleotide Polymorphisms or SNPs). Using these mutations, the test can give more granularity and specificity to your relationship to your matches. You will be assigned a haplogroup (tree branch) that will likely be within the past 100-500 years or so. Using this information, you will be given a time estimate for a birth year for your common ancestor with your other branch mates. In Jewish genealogy, we use the term “lineage” to talk about these relatives who share a paternal ancestor, since Jewish surnames were not regularly in use in Europe prior to the 19th century. A lineage becomes a sort of proxy for surname in this case.
About 600 Y-DNA haplogroups have been identified as having a Jewish paternal line ancestor. If the testers on your match list have identifiably Jewish surnames or their earliest known ancestor has an identifiably Jewish name, then you likely belong to one of these known Jewish haplogroups.
Note that if one person from the paternal line you’re studying has tested Y-DNA, it’s not necessary or useful to test other close (3rd cousin or closer) paternal line relatives unless you’re researching possible misattributed parentage.
Only people born with a Y chromosome can be tested for Y-DNA.
Mitochondrial DNA is a different type of DNA that exists outside of the nucleus of the cell in the mitochondria, so it’s not part of the 23 pairs of chromosomes that are compared in an autosomal test. Mothers pass mtDNA to their children unrecombined, both sons and daughters, but only genetic females pass it on to their progeny. Of the major DTC testing companies, only FamilyTreeDNA offers mitochondrial DNA (also written as mtDNA) testing. The mtDNA test sequences the full mitochondria, since it contains a relatively small amount of DNA compared to the 23 chromosomes within the nucleus.
Your match list will show testers with whom you share a common ancestor on your maternal line (mother’s mother’s mother’s mother, etc.) This type of DNA can be somewhat problematic to use for genealogical research due to the difficulty of tracing female ancestors. However, your mtDNA match list can give you an indication of Jewish ancestry or not on that line. If the majority of your matches have identifiable Jewish surnames or maternal names that are identifiably Jewish, then you likely belong to a known Jewish mtDNA haplogroup.
Note that if one person from the maternal line you’re studying has tested mtDNA, it’s not necessarily or useful to test other close (3rd cousin or closer) maternal line relatives unless you’re researching possible misattributed parentage.
Anyone can be tested.