Newman says that there’s a basic lack of “literacy” and understanding about genetic testing, among the public and even other health professionals. People are given false reassurances or made to panic (just because you have certain genetic variants, it doesn’t mean that you will develop a particular condition). Newman also makes the point that, in his field, counselling happens before and after testing and, while people with cancer or heart issues nearly always opt to have the test (as they can then take action to varying degrees), often people with conditions such as Huntington’s disease in their family decide not to go ahead because a diagnosis would change nothing for them. In any event, Newman says that, with genetic testing, while there are different levels, intensive counselling is always “absolutely key”.
Is this a perfect method?  No, but it’s a good way to get a general idea about where your ancestors were from.  Genealogical DNA tests can tell you a lot about your ancestry going back 300-500 years in time, for the most part.  They can also tell you a little bit about your ancestry going even further back.  This is why comparing your DNA to those whose families have stayed in a particular area for a long time is a fairly accurate way to perform the estimate.
So, back to the example of the two men (lets call them John and James) we think might be brothers: We find two g-g-grandsons (or any male descendant in a direct father-son line) of each, preferably the most distant cousins we can find, and get a Y-DNA test done for all four men. Usually a 37 marker Y-DNA test is a good place to start. This looks at 37 sections of each persons DNA that the genetic scientists think are most useful for our purposes (the bits that are least likely to randomly change over generations). The results come back looking like a fairly meaningless string of numbers which are fairly useless on their own, but allow us to compare each persons result with others in the database of the testing company.
DNA test companies that use genotyping technology, including 23andMe and Ancestry, allow you to download your raw DNA file. A raw DNA file is usually a text file that contains all the information about your genetic code gleaned from the company’s examination of your DNA. This is comprised of several hundred thousand markers known as SNPs (Single Nucleotide Polymorphisms). Most raw files are organized into five columns: the SNP coded into an rsID number, the chromosome the SNP is located on, the location of the SNP on the chromosome and the two alleles for each SNP.
When asked about how database size affects ancestry results, David Nicholson, co-founder of Living DNA, told us, “The tests absolutely rely on the reference database. If you have Polish ancestry but there are no people in the database who are Polish, then what the test will do is show what the next closest group is next to Polish, like German or Eastern European ancestry.” 
1. Family Tree DNA’s Family Finder: One of the first to offer these types of tests, FTDNA is generally considered the leader in autosomal DNA testing for ancestry and provides some of the best tools available for genealogists. Their population finder section is currently a bit less specific than the other companies, but FTDNA Family Finder users are eagerly awaiting a much improved update to be released sometime in the next month. Find out about Family Finder here.