Friday, January 23, 2009

DNA uncovers your geographic origen

An European woman, a Yoruba man, a Japanese child. Which historical and demographic events explain the genetic differences between human populations? Two of the genetic markers that are most used to study human migrations are the Y chromosome and mitochondrial DNA (mtDNA), a type of DNA found within the cellular organelles called mitochondria. These are the only two types of DNA that do not undergo genetic recombination, a phenomenon in which the DNA sequences coming from the mother and the father mix up. The Y chromosome is transmitted intact from the father, and the mtDNA comes always from the mother. The fact that they come always from one parent only makes it much easier to follow their lineage.

Thanks to these markers we know about the large human migrations, such as the colonisation of America and the islands of the Pacific, or the migration out of Africa that took place about 60,000 years ago, first to Asia and Australia and then to Europe and the American continent. We also know that the mtDNA is globally more homogeneous than the Y chromosome. This implies that, contrary to what is usually assumed, women have migrated more than men. This global migration pattern can probably be explained by many small migrations due to the fact that women used to go to live to the regions where their husbands were from.

But there are still many local migrations to understand. For example, the research group on evolutionary biology at the CEXS-UPF has recently done a study on the Cuban population in order to see if there was an Amerindian genetic footprint left. They were the original population of the island, who were exterminated during the European colonisation. The results show that, even though there are no Amerindian markers in the Y chromosome, up to a third of the genetic background of the mtDNA is Amerindian. This implies that the colonisers had sexual relations with the Amerindian women before their total extermination.

And why is it interesting to study migrations? Apart from the purely historical interest, it is important for genetic epidemiology studies to know the structure of the current populations. For example, to determine if a common genetic variant in a population is associated to a disease it is necessary to first know whether this population is homogeneous.

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