Every company providing DNA testing has their own database of DNA samples. These are called AIM’s or ancestry informative markers. These markers were derived from the current populations of America, Asia, Europe and Africa. Every database is looking for a pair of genes located on a specific chromosome in a specific position. The way the DNA is evaluated is through the SNP’s or the single nucleotide polymorphisms. The SNP’s are chosen according to the frequency in a specific geographical population. Your SNP’s are compared to the most common SNP’s for the various populations in the company’s reference database. The results are not conclusive because they are based on common genetic variations. The probability for your DNA being from a certain country is based on a comparison between your DNA and the database. If you used a different company, they would have a different database. This means your results would most likely differ. According to studies, the lowest concordance is with individuals of South Asian, East Asian and Hispanic descent.

Hi, I’m just a bio major, but my genetics professor was just talking about this. These tests are not complete genome sequencing. They only look for SNPs (genetic markers) parts of the genome that tend to have a lot of variation. They determine your ansestry by comparing your markers to those of others in their database, but because their database is always expanding, the numbers change. Its still a good approximation and gets more accurate as more people take it, but if you want to be certain you have to have complete genome sequencing. There is a company called Nebula that does it. Hope that helps.
More controversially, some of these kits profess to tell you your biological (as opposed to actual age) by measuring the length of your telomeres (in basic terms, the caps at the end of each strand of DNA that protect chromosomes, like plastic tips at the end of shoelaces). Other tests, such as 23andMe, predict higher risks of developing serious conditions, such as multiple sclerosis, Alzheimer’s and Parkinson’s, including the test for BRCA1/BRCA2 (breast and ovarian cancer) that Angelina Jolie famously underwent, going on to have a preventative double mastectomy and surgery to remove her ovaries.