Genetic studies make it possible to better identify certain predispositions to hereditary diseases. They also help historians to assess the importance of historical migrations in geographical areas that have hitherto remained unclear. How many Britons migrated from Great-Britain to Armorica in the 5th century AD ? A biology journal has published a global study of genetic inheritance in France, work carried out in close collaboration between two university teams, one in Brest (UMR 1078 Inserm-UBO) under the direction of Dr. Aude Saint Pierre , and the other, the team of Dr. Christian Dina at the Institut du Thorax in Nantes. This study is the first to draw up a genetic map of France. The study identified six groups in France. They correspond both to relatively clearly delimited geographical areas or to regions which have often had a different history such as Brittany, or curiously, to linguistic areas such as Occitania. The study involved 2184 individuals using rigorous scientific methods. The genome variations studied are based on the Single-nucleotide polymorphism or SNP. The territorial unit of the study is the department, each department has a circular diagram called cluster (see map). Clusters of course have several components but often with a dominant component. On the map, the Breton cluster with a dominant single pink component shows a correlation with the historical area of the Duchy of Brittany. The pink component has a surprising outgrowth in the Mayenne and a spin-off towards the east as far as Paris. Note that the Loire-Atlantique department is as rosy as Ille-et-Vilaine. The Occitan cluster has a single dominant yellow component, the Basque-Gascon cluster has an equally unique brown component. Other studies showed that the Breton cluster has haplotypes (see article) that were also found in Great Britain. We are awaiting a study at European level to confirm this.