Joana Barbosa Pereira, human population researcher
Institute for Research and Innovation in Health (i3S)
Ancestry testing brings together interests in family history and genetics, a trend that is here to stay. But few people understand how ancestry testing works. In this article, we explain how a small sample of saliva can give us immense information about an individual’s genetic history and how the size of databases can change details about our ancestry. Nevertheless, genetics is only a small piece of the puzzle of who we are.
We all inherit our genetic information from our parents, so in fact we are actually inheriting parts of our ancestors from many generations ago. As mutations are passed on to subsequent generations, they have accumulated over time generating variation between groups of individuals, populations, or species, which become greater as time passes. This means that people within a given region tend to be genetically closer related than people living further away. This variation is written in our genetic code as our DNA consists of four letters – A, T, C, and G – each representing a chemical building block that together forms a unique sequence of a person of more than twice 3.2 billion bases. In an ancestry test we are zooming in on the similarities and differences within and between populations and the great advantage is that we don’t need to know the exact sequence of letters in the whole genome to understand our ancestry but screening a few thousand positions that are known to be heterogeneous in frequency between populations or groups are enough. Although a few thousand may seem a lot, it represents less than 0.01% of our DNA.
As the majority of you probably know, the first step for ancestry tests is the self-collection of saliva, from where researchers will extract the DNA. Then, we genotype the samples – which is the process used to determine the DNA sequence at specific positions within the genome using a determined set of markers. These markers vary according to the different companies and chips used, but most include analysis of the autosomal markers, Y chromosome, and mitochondrial DNA. The autosomal analysis allows for a more in-depth search through all of the nuclear chromosomes, equally transmitted by the mother and the father. Y chromosome and mitochondrial DNA testing allow the tracing of patrilineal and matrilineal lines, respectively.
Conclusions are drawn by comparing an individual’s DNA profile to a reference database of thousands of other people whose ancestry is known and who represent ethnically and geographically diverse populations. This admixture analysis generates a rough estimate of an individual’s ancestry – you cannot be too literal in interpreting your results since the reference sets and algorithms variation have associated margins of error. One thing is sure – most human individuals will have an admixed profile as human populations have been sharing genes throughout their existence. Some populations are better represented in reference databases, making it easier to correctly assign an individual ancestry to a specific geographical location. For instance, while European populations are well represented in reference databases, allowing a high accuracy across the continent, fewer African populations have been genotyped, hence we have lower accuracy in placing an African individual on the African continent. Other populations just share many ancestors over time, such as Portuguese and Spanish, which makes it difficult to distinguish which population an individual is more likely to belong to.
Genetic ancestry is just a small piece of the puzzle of who we are and where our ancestors came from. However, ancestry reporting is constantly evolving as DNA databases grow and the availability of genetic testing at ever-lower costs increases.
