Correct, you can measure heritability without any genetic information whatsoever. Because of this we can know that, say, 60% of Vit D variability is heritable.

Correct, you can measure heritability without any genetic information whatsoever. Because of this we can know that, say, 60% of Vit D variability is heritable. I hadn't seen a GWAS on Vitamin D levels before you asked your question, but I pubmedded it and it turns out that a small one came out this month: http://hmg.oxfordjournals.org/cgi/content/full/19/13/2739?view=long&pmid=20418485 http://hmg.oxfordjournals.org/cgi/content/full/19/13/2739?vi... In short, 4 base pairs (out of 3 billion) account for 2.8% of the population variance of circulating Vitamin D. With such a small # of people, it's not surprising that few independent SNPs hit the genome-wide significance level, but it's still pretty impressive that the 4 SNPs that they found can account for so much of the variance. (In other words, it looks like they identified the source of ~5% of the genetic variance, or 2.8% of the total variance.) This looks much like the lipids story: early GWASs give you a handful of loci with influence, and later GWASs with may more participants will give you a much richer set of loci that collectively explain 20-30% of the variance. And keep in mind that these are just the common SNPs.