Global vs Local Ancestry: Reading the Map vs Zooming into the Streets

When we talk about ancestry, most people think of those colourful pie charts showing what percentage of your DNA comes from different parts of the world maybe 40% African, 30% European, 20% Asian, and a sprinkle of something else. That’s your global ancestry, the big picture view. You can think of global ancestry like looking at a world map from space. It tells you which continents your ancestors came from on average, but not the details of where each piece of your DNA actually comes from.

Now imagine you zoom in; street level, house by house. That’s local ancestry. Instead of seeing your ancestry as one big mix, local ancestry looks along your genome, chromosome by chromosome and even down to specific genetic regions, to see which ancestry each section of your DNA came from.  

For example, one stretch of your DNA might be inherited from an ancestor with African roots, while a nearby stretch could trace back to European or Asian origins. Your genome becomes a patchwork quilt of ancestry segments, each telling a small part of your story.

In genetics research, this difference matters a lot. Global ancestry is great for general patterns, for example, showing that a population has 60% African and 40% European ancestry on average. But local ancestry helps researchers understand how that ancestry is distributed across the genome and that’s where things get exciting.

Let’s say scientists are studying a genetic variant linked to a disease. If that variant appears mostly on sections of the genome inherited from a particular ancestry (say, African segments), that tells them something powerful: the variant may have originated, or acts differently, in that ancestral background.

This is especially important in admixed populations, like many found in South Africa, where individuals carry a mosaic of ancestries. Local ancestry analysis helps researchers disentangle how these ancestral pieces contribute to health, disease risk, or even how certain medicines are processed in the body.

So, while global ancestry gives us the broad strokes of who we are, local ancestry paints the fine details. One tells the story of our shared roots; the other helps pinpoint how those roots shape the DNA we carry today, one chromosome at a time.