From the 4th century the Roman Empire is starting to have trouble containing the hordes of Germanic tribes at its borders.
As early as 288, the Franks are allowed to settle in the Germania Inferior region of Gallia Belgica.
The Huns from central Asia advance westwards, forcing entire populations to follow the move. The Alans, from the northern Caucasus, settled in Pannonia (modern Hungary and around).
In 376, the Visigoths are tolerated in Gaul on condition that they defend the borders of the empire; but they rebel and sack Rome in 410 and start ruling the empire.
In 406, the Alans, Vandals, Suebi and Burgundians cross the Rhine and settle in Gaul. The first Burgundian kingdom appears in 411 in eastern Gaul. The Alans, Vandals and Suebi move to Iberia. The Suebi set up a kingdom in Gallica, while the Vandals move to North Africa.
During that time, the Angles, Saxons and Jutes invade Roman Britain.
In 493, the Ostrogoths, invited by the Byzantines to recover Rome from the Visigoths, invade Italy and create the Ostrogothic kingdom, pushing the Visigoths to southern Gaul and Iberia.
At the same moment, the Lombards settle in the Danube valley around modern Austria. In 568 they defeat the Ostrogoths and become the new rulers of Italy.
The Huns, after roaming through Europe, siding with one Germanic tribe or another, eventually settle in what is now called Hungary.
Reading a history book, one could think that the massacres committed during this period of wars and destructions left plenty of space for the invaders to spread their genes. They created new kingdoms that would evolve in medieval kingdoms, spawning many noble families, who usually had more children than poor peasants working for them.
It has even been claimed that the Anglo-Saxons never intermarried with non-Germanic folks, and indeed exterminated most of the Romano-Britons and pushed the survivors away to Wales and Scotland.
But how much of this is true ? For the first time we are able to find out the real answer, whatever history books tell us or don't tell us. Thanks to Y-DNA tests, it is possible to know the percentage of male Germanic lineages in each region of Europe.
=> see Y-DNA haplogroup frequencies in Europe by country
The only thing that is not obvious from the table is the percentage of Germanic R1b. R1b is mostly a Celtic haplogroup, including ancient Italic, Iberian, British and Irish people. But one branch, known as U106 (or S21) originated in the Netherlands and northern Germany, homeland of the Frisians, Franks, Angles, Saxons and Lombards.
Germanic ancestry can therefore be traced by looking at the R1b-S21, I1 and I2b haplogroups. I have calculated the approximate percentages by country.
We can see that the only parts of the Roman Empire that have been massively settled by Germanic people (at least one third of the modern population) are England, the Benelux, northern and eastern France, southern Germany and Austria. This corresponds to the regions where Germanic languages are now spoken, or used to be spoken until a few centuries ago (Nord-Pas-de-Calais, Alsace, Lorraine).
Nevertheless it is very clear that Germanic people did not replace the existing populations. In England, only half of the lineages are truly Germanic. The percentage is higher on the east coast, but this is also because the Vikings settled in great number there. Without the Vikings, England might now just be 1/3 Germanic.
Austria and south-west Germany are surprisingly only one third Germanic themselves. In Baden-Württenberg, about 60% of lineages are Celtic, and at least 10% of Greco-Roman origin. Switzerland is even less Germanic (22% according to the data available to me). Interestingly, almost all R1b is Austria is the S21 variety, which let us suppose that the Lombards settled massively in the region.
Italy was invaded by at least 5 Germanic tribes (Visigoths, Ostrogoths, Vandals, Lombards, Franks), and yet less than 10% of modern lineages are Germanic. Most are to be found close to the Alps (or in Sicily, but that is due to the later Norman invasion).
Iberia was the prey of the Visigoths, Vandals and Suebi, but they contribution to the modern Y-DNA haplogroups is a mere 5% of the population. The Vandals seem to have had the smallest impact, probably because they didn't settled in Iberia. Even in regions of the old Visigothic kingdom, in Catalonia and south-west France, only about 2% of modern lineages can be traced back to them. The Visigoths may have settled more around Aragon and Valencia. It is the Suebi who have had the biggest impact, judging by the slightly higher percentage of Germanic haplogroup around their old fief, in Galicia, north-western Iberia.
As for the Huns from Central Asia, it is though that their genetic legacy is associated with haplogroup Q. It is bold of Hungarian people to think of themselves as the descendants of Attila. Indeed, only 3% of them belong to haplogroup Q (and 2.5% in neighbouring Slovakia, part of which was historically Hungary). Ukrainians are more Hunnic, with 5% of Q.