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Estimating geographical ancestry from DNA

Identification through DNA profiling can only occur when there is a reference profile for comparison, obtained either from a suspect in custody or from the criminal database - in the case that the person of interest has been previously convicted. However, in a large number of forensic investigations, there are no suspects or database matches and, when no leads other than DNA evidence are available, there is an urgent need to obtain more information from DNA. The forensic field that focuses on obtaining information from DNA that can help narrow down the list of potential people of interest is known as ‘DNA Intelligence’ and it mainly relies on DNA sequencing analysis.

One of the main types of information that can point an investigation towards a person of interest is the estimation of their geographical ancestry. While it is well-established that human populations originated from Africa, centuries of migrations, followed by population mixing as well as isolation of certain groups, have formed the current separation of the world population in groups with distinctive biogeographical ancestries. At the same time, small mutations in the human DNA that consist of single nucleotide changes and are known as ‘single nucleotide polymorphisms’ (SNPs) have followed the flow of human populations passing from one generation to the next.

An example of this phenomenon is the SNP associated with sickle cell anemia. When these single nucleotide changes from adenine to thymine is present in both chromosomes of a person, it results in red blood cells adopting a particular shape under certain conditions (like high altitude) that affects their ability to transport oxygen. However, people heterozygous for this mutation that have one chromosome with the normal nucleotide (adenine) and one with thymine not only do not show any symptoms of the disease, but also demonstrate 10 times higher resistance to life-threatening forms of malaria. As this trait massively increased the survival rates of heterozygous people in areas with high risk of malaria, it is still found in much larger percentages in populations originating from there than from any other region of the world.

A number of similar SNPs have been incorporated in panels that have been developed by different laboratories around the world and tools for the accurate estimation of geographical ancestry are now available to forensic analysts and have been recently employed in a number of cases.

Alt text for screen readers A heatmap demonstrates which continents have the least resistance to malaria. Resistance to malaria is low globally, but extremely concentrated in Africa.

Discussion prompt

Can you think of any ethical issues that might arise from this approach?

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The Science Behind Forensic Science

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