Why do DNA results differ between services?
Your DNA is the same — but the file describing it is not. Three technical reasons explain almost every difference.
1. Different chips = different marker sets
Every provider uses its own test chip measuring a specific subset of SNPs. MyHeritage, 23andMe, AncestryDNA and FamilyTreeDNA overlap substantially — but not completely. A link built on an rsID present in one export but not the other will show in the first case and be marked as missing in the second. That is not an error — it is the coverage of the files.
2. Different reference versions (build)
Genome positions are given against a human reference sequence. Older files (FamilyTreeDNA's in particular) can use Build 37 while newer data refers to Build 38. The same rsID points to the same biological position, but the coordinate number in the file differs — see Build 37 vs 38 explained. An analysis that does not normalize for build could in theory read the wrong position — DNAHelper matches on rsID, which is build-stable, rather than raw coordinates.
3. File formats and reporting conventions
The files look different — MyHeritage and 23andMe are tab-separated text files, Ancestry a semicolon- or tab-separated CSV, FTDNA a compressed CSV — and no-call rows (failed measurements, often --) are handled differently. A service can also filter markers at export, or change which allele is listed first. All of it affects what an analysis can read.
What does not differ
Your actual genetics are constant. If the same SNP is measured by two files you get the same genotype in both — the difference is which markers are covered, not what they say. And the interpretation model is the same: associations with evidence grading, not diagnoses. How we grade evidence.
Practical rule
Tested with several providers? The file with the most relevant markers gives the most results — try both, they are free to analyze. Our format comparison describes what each export contains.
Also: What is a SNP? · What is imputation? · Build 37 vs 38 · File formats by provider · English overview