Mistaken zygosity is quite rare, in the single percent range, after doing some tests like blood groups or hair whorls. In this case, the DNA overlap is vastly h

Mistaken zygosity is quite rare, in the single percent range, after doing some tests like blood groups or hair whorls. In this case, the DNA overlap is vastly higher than would be expected for siblings: > According to the raw data from 23andMe, 99.6 per cent of those parts were the same, which is why Gerstein and his team were so confused by the results. They concluded the raw data used by the other four companies was also statistically identical. If they were siblings, ~50% of the (several hundred thousand) SNP calls would be the same, not 99.6%. (Which is about what you would expect from identical twins, since that's where either one disagrees with the other, and errors usually don't strike twice, so a 99.6% overlap roughly implies a per-SNP accuracy rate of sqrt(0.996) = 99.8%, which sounds reasonable given the difficulty of sequencing and the inherent noise & randomness.) On a fun side note, identical twins are not exactly 100% genetically identical, which is how rapist/murderer identical twins are being convicted these days based on DNA evidence; but there are so few unique mutations to each twin, and the new mutations wouldn't be on the common-SNP tests, that you have to do the equivalent of like 4 whole-genome sequences (IIRC, you need coverage of ~120x vs the more usual WGS of 30x) to get enough evidence to override the prior of any difference being just a repeated sequencing error, and it's very expensive.