We just published a typical GWAS paper that used PCA to sanity check whether the "ethnicity" reported by our patients aligned with what their genome told us.

We had 200,000 dimensions (ACGT's), which we reduced into 2 via PCA and sure enough if someone said they were "Filipino" then they generally appeared close to the other folks who said they were "Filipino".

https://breckuh.github.io/eopegwas/src/main.nb.html (chart titled: QC: PCA of SNPs shows clustering by reported ethnicity, as expected)