So when the companies look at the 600,000 or so SNPs they use in our DTC tests, they infer what the untested, missing SNPs probably are based on our genotypes, benchmarks based on standardized genomes, even though those SNPs were never tested. There are around 25 million SNPs that any two of us might differ on, and within similar global populations-say, Northwest Europeans, for example-that per-person difference may be as small as 4 or 5 million SNPs. All humans have DNA that is a fraction over 99% identical. But it works for genealogy thanks to something called genotyping. Our DTC tests look at only about one DNA letter in every 5,000.

STRs can't even be guessed at from the raw data in our genealogy tests. That's why STRs are used in forensics and paternity cases, and never our SNP tests done on a microarray chip. STRs will change by-generation far, far faster than the SNPs (single nucleotide polymorphisms) that our DTC tests look at. That means (aside from a possible current-generation mutation, or change) an inherited pattern of 10 repeats came from one parent, and 12 repeats from the other. When test results come back, D5S818 will show two values, like 10-12. That ATCT pattern is known to repeat, one set right after the other, from 7 to 17 times. It's on Chromosome 5 and consists of a sequence of four DNA letters: ATCT. Each STR consists of an identical set of, generally, 2 to 7 alleles-the DNA 'letters' A, C, G and T-that repeat in exactly the same, known pattern multiple times at an identified place on a specific chromosome.įor example, one of the core STRs used in forensics in the U.S. And it's highly unlikely they ever will be admissible for any legal purpose.Īll forensics testing right now examines what are called short tandem repeats (STRs). I don't know whether it will set your minds at ease or not, but the fact is that none of our common direct-to-consumer DNA tests can be used in any court of law I'm familiar with, certainly not in the United States or the United Kingdom.

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