Hi, Sandy and Tiffany.
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. And it's highly unlikely they ever will be admissible for any legal purpose.
All forensics testing right now examines what are called short tandem repeats (STRs). 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.
For example, one of the core STRs used in forensics in the U.S. is called D5S818. It's on Chromosome 5 and consists of a sequence of four DNA letters: ATCT. That ATCT pattern is known to repeat, one set right after the other, from 7 to 17 times. When test results come back, D5S818 will show two values, like 10-12. 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.
STRs will change by-generation far, far faster than the SNPs (single nucleotide polymorphisms) that our DTC tests look at. That's why STRs are used in forensics and paternity cases, and never our SNP tests done on a microarray chip. STRs can't even be guessed at from the raw data in our genealogy tests.
Our DTC tests look at only about one DNA letter in every 5,000. Pretty spotty stuff. But it works for genealogy thanks to something called genotyping. All humans have DNA that is a fraction over 99% identical. 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. 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. When we see a DNA segment reported that's 20cM in length starting at position X and ending at position Y, that's never entirely accurate because we looked at only a few thousand of the markers along that physical segment which could be well over 20 million markers long.
That's why our DTC tests are not, and almost certainly never will be, used for forensics in a court of law. I believe cases like the Golden State Killer and Cece Moore's television show, The Genetic Detective, have caused some confusion about that...as shown by the slowdown in DNA test sales growth numbers following the GSK case publicity in 2018.
But Cece and company never provide court-admissible, convicting DNA evidence. She can't. What she does do is perform the same kind of genetic genealogy research we all can, e.g., if I know who Person A is, and there's a match in unknown Person B, let me try to trace the family trees to see if I can guess who B is.
In one of my trademarked World's Worst SimilesTM: It's like law enforcement knowing where and when a crime was committed, and that a car was most likely employed in the crime. Cece will go in and use her analytic skills to uncover that a Toyota Corolla with a license plate starting with an "S" and ending with a "35" was probably in that place at that time. What she uses as genealogical evidence never goes to court. But law enforcement can then decide whether they want to act on the clue provided and obtain an admissible DNA sample that can be subjected to an admissible CODIS-standard STR test. That's what is submitted as the DNA evidence.
Whew. A tour you never asked for. And with all that said, as technologies continue to improve and the effective costs lowered for high-throughput, high-accuracy, long-read whole genome sequencing, it may eventually supplant today's STR tests for forensics. But that will be a long time coming. We'll be using the technology for genealogy long before law enforcement...because they've been amassing databases of STRs for over three decades, and because such a change across law enforcement agencies will, bureaucratically, take a very long time and cost a very pretty penny.