rs12785878
How much vitamin D your skin can make
What this position does
DHCR7 controls the supply of the precursor your skin turns into vitamin D under ultraviolet light. Variants here rose in frequency in northern populations, which is what you would expect where sunlight is scarce for half the year.
Skin. Shaped by making vitamin d from weak northern sun. This is one of the variants that differs sharply between human populations, because something in the environment made one version worth having.
Sunlight and vitamin D
Skin has to let in enough ultraviolet light to make vitamin D, and block enough to protect the folate in your blood. The right answer depends entirely on where you live, so it changed every time people moved.
What changed: Industry, roughly the last two centuries. Indoor work, heated buildings, refined food, cheap salt and sugar, tobacco at scale, and a collapse in infectious disease. This is where most of the deals in your genome came undone.
Why it spread: well established. The environmental pressure behind this variant is documented and not in serious dispute.
Spiral Staircase, chapter 19: The Largest Organ Spiral Staircase is written by the same author as this site.
What each result means
| If you carry | What it means |
|---|---|
| No copies of the effect version GG | You have the lower-vitamin-D version on both copies Associated on average with lower measured vitamin D. |
| One copy GT | You carry one higher-vitamin-D copy Associated with somewhat higher measured vitamin D. |
| Two copies TT | You carry two higher-vitamin-D copies Associated with the highest average measured vitamin D at this position. |
Worth knowing. Latitude, season, skin tone and time outdoors all matter more than this. A blood test settles it.
How common each version is
| Ancestry group | T (the effect version) | |
|---|---|---|
| African | 17.0% | |
| American | 45.0% | |
| East Asian | 38.0% | |
| European | 70.1% | |
| South Asian | 15.1% |
1000 Genomes Project phase 3, via Ensembl, retrieved 2026-09-23. These groupings are coarse and flatten a great deal of internal diversity, particularly within Africa. A variant being common somewhere tells you where a piece of your ancestry probably comes from, and nothing else.
How many people carry each result
| Ancestry group | GG | GT | TT |
|---|---|---|---|
| African | 456 / 661 | 185 / 661 | 20 / 661 |
| European | 49 / 503 | 203 / 503 | 251 / 503 |
| East Asian | 190 / 504 | 245 / 504 | 69 / 504 |
| South Asian | 358 / 489 | 114 / 489 | 17 / 489 |
| American | 111 / 347 | 160 / 347 | 76 / 347 |
Counts of people, not modelled frequencies: every one of the 2,504 individuals sequenced by the 1000 Genomes Project phase 3, tallied by the genotype they carry. Deriving these from allele frequencies instead would assume random mating inside each group and undercount the homozygotes.
How to find out which you have
If you have taken a test with 23andMe, AncestryDNA, MyHeritage, Family Tree DNA, TellMeGen or Living DNA, this position is very likely already in the raw data file they gave you. You do not need another test.
One thing to watch if you look it up by hand: testing companies report some positions from one DNA strand and some from the other, and the file does not say which. Read the letters as they appear and you will get the opposite of the truth for some positions. Sequencings resolves that automatically.
Source
Wang et al., Common genetic determinants of vitamin D insufficiency, The Lancet (2010)
Coordinates and allele pair verified against Ensembl GRCh37. Direction of effect verified against VEP protein consequences or 1000 Genomes population frequency. Category: Population adaptation.
Where this fits
Common questions
What is rs12785878?
rs12785878 is a position in the DHCR7 gene on chromosome 11. DHCR7 controls the supply of the precursor your skin turns into vitamin D under ultraviolet light. Variants here rose in frequency in northern populations, which is what you would expect where sunlight is scarce for half the year.
What does it mean if I have TT at rs12785878?
You carry two higher-vitamin-D copies. Associated with the highest average measured vitamin D at this position.
What does it mean if I have GG at rs12785878?
You have the lower-vitamin-D version on both copies. Associated on average with lower measured vitamin D.
How do I find out my rs12785878 genotype?
If you have taken a home DNA test, this position is very likely in the raw data file you were given. Sequencings reads that file in your browser without uploading it, and resolves which DNA strand each position was reported from, which is the step most manual lookups get wrong.
This is reference information, not medical advice, and not a diagnosis. See the medical disclaimer. Category: Population adaptation.