8 - Probing Your DNA

Summary

When you purchase a DNA ancestry service you are sent a kit containing instructions for submitting a DNA sample. Most companies provide a plastic tube, which they ask the test-taker to fill with saliva or cheek swabs, seal, and return. When the company receives your DNA test sample it is processed for analysis. As noted previously, the vast amount of your genome does not distinguish you from other individuals. Therefore, your genome is broken down into segments of DNA that contain the alleles of interest, rather than it being fully sequenced. Here is how AncestryDNA describes the processing of the DNA samples it receives from customers:

[T]o obtain a customer’s ethnicity estimate, we divide the customer’s genome into small windows. For each window we assign a single population to the DNA within that window inherited from each parent, one population for each parental haplotype. Each window gets a population assignment based on how well it matches genomes in the reference panel.

We do not know the exact haplotype boundaries, which differ between people, but we can achieve a good approximation by dividing the genome into 1,001 small windows. Each window covers one section of a single chromosome and is small enough (e.g., 3–10 centimorgans) that both the maternal and paternal haplotype, the DNA from Mom and the DNA from Dad, in a given window are likely to each come from a single, though not necessarily the same, population.