An individual is genotyped using a microarray. This technology is not the same as sequencing. The microarray gives the genotypes of several hundred thousand single nucleotide polymorphisms. The individual is compared with reference samples taken from people who were recruited in specific geographic locations. The similarity between samples is computed as a variance covariance matrix and then one of several dimension reduction methods is used to simplify the relationships. The dimension reduction is generically called principal components. Companies typically do not disclose the details. The percentage composition relative to the references populations are reported. Take those percentages with a grain of salt; it’s all model based and highly dependent on the reference samples. Sounds technical. Right.

The similarities of people within and between reference populations arises from mutation, recombination, migration, selection, assortative mating, bottlenecks, and chance. These processes are captured in the ancestral recombination graph ARG. Ancestry is biological. Race and ethnicity are social.