23andme tests the entire genome which is 22 chromosomes and the X and the Y and mitoDNA.A male has one X and one Y.Everyone has mitoChondrial DNA which they inherit from their mother, only females pass on their mitochondrial DNA (mtDNA).Females have 2 Xs,one from each parent.Men get one only from their mother..
The test is Autosomal which means across the whole genome and arrayed are 2 of each chromosome ( maternal and paternal).They are in tiny tiny pieces called SNPS which is different from older tests on mitoDNA and Y DNA tests which are in STRS. STRS are bigger chunks of DNA.
These autosomal snps are in clusters inherited through your ancestors and are specific to YOU and your family.Anyone who you share a segment with is related to YOU, the larger the segment the more closely related.Smaller segments of less than 6.5cM are thought to indicate an ancestral place or a very distant ancestor and shared by many more people.
I now have 1082 cousin matches,some are my first cousins who I knew about. Others are startlingly with people I never knew about who descend with me from John Carrow’s line 1660-1750; Charles Swanson in 1809;my many Irish ancestors and a few cousin from the same small village north of the Arctic Circle where my grandfather Sigurd Boe was born.
23andme also tests health traits for the same $99 and they are also available to you without charge. Every time new tests get completed new cousin matches and health traits are sent to you.
I have tested Family Tree DNA's Family Finder, 23andme and DeCodeME ( by upload). I have not tested at AncestryDNA but it is available to me via a cousin and it is not as good as the other two.
I feel 23andme is the BEST autosomal test It has the largest database at about 200,000 and are now in a growth phase to reach 1 Million.. Family Tree DNA has about 35,000 and AncestryDNA has slightly more than 50,000 ( they have given many free tests out)I am an Administrator of 5 surname groups at Family Tree DNA, 3 Ancestor Projects at GEDmatch.comand 5 at Ancestry.com.
Gedmatch.com and others are called “third party” comparison sites. They are available to folks for free.You must have access to your Raw Genome to transfer your data to the third party sites.Ancestry.com currently does not allow you access to your own genome so transfers cannot happen.
Family Tree DNA allows upload of 23andme genome for a transfer fee of $89. You must have a V3( newer test) to do that upload.23andme does not upload FTDNA data at this time. AncestryDNA used to allow you to manually put your test results from other companies into their groups but that is no longer being done.
DEcodeME is an Icelandic company that allowed genome transfer but they have been purchased by another company.