To visualize population substructure for the genetic data, a principle component analysis was conducted (Supplementary Text 3). There was a clear correspondence between the first two principal components based on the genome-wide single-nucleotide polymorphism genotypic data and parental country of birth as registered in the Danish Civil Registration System (Figure 2). Individuals born in Denmark to parents born in other Scandinavian countries clustered together with Danish-born individuals with Danish-born parents, as expected. Within each foreign parental region of birth, individuals with two foreign parents were as anticipated more genetically divergent compared to those having only one foreign-born parent. This finding provides strong evidence of internal validity for processing of individual samples and the ability to link to information in the rich Danish registers on an individual level.
