Razib Khan, discussing the reliability of commercial DNA ancestry tests, describes the genetic links between Germans, Scandinavians, and Finnic peoples:
Scandinavia is a coherent ethnolinguistic category which encompasses various northern Germanic people who were relatively untouched by Roman cultural influences. This is in contrast to many Germanic tribes to the south, such as the Franks, who emerged in dynamic tension with the rise of the Roman Empire. The final Scandinavian conversion to Christianity, and so admission into the post-Roman European world, began about two centuries after the conversion of the pagan Saxons by Charlemagne.
Later, the two centuries of the Kalmar Union brought all the modern nations of Scandinavia under one ruler. Today, the concept of Norden, which includes non-Scandinavian Finland, expresses the cultural and social connections of the northern peoples.
And yet genetically the reality is more muddled. Looking at samples of Germans, Danes, Swedes and Norwegians, the geographic patterning is clear. Danes occupy a position between Germans on the one hand, and Norwegians and Swedes on the other. Because of Sami ancestry in many Norwegians and Sami and Finnish ancestry in many Swedes they are genetically distinct from continental Germanic peoples to the south, including Danes.
So what is a Scandinavian? A Scandinavian is a Swede, Dane, or Norwegian (or an Icelander). Scandinavians share 1,000 years of history since their integration into the European system. As a cultural category Scandinavians are clear and distinct.
But as a genetic cluster things are not so clear. First, there is the Danish connection to Germany. This is due to both history and geography. People from northern Germany are clearly genetically close to the Danes. While the Angles and Jutes were from modern Denmark, the Saxons were from northern Germany. Yet in Britain, they fused seamlessly into one people. Before the mass conversion of the continental Saxons under the Carolingians, the cultural barriers between the peoples of Jutland and Saxony must have been marginal at best.
Second, an enormous number of Swedes in particular seem to be highly admixed with Finnic peoples. Many Swedes are highly “Finn-shifted”, both due to Sami assimilation in the past few hundred years, and the long history of Finnish migration into Sweden (which dominated Finland either politically or culturally for nearly 1,000 years). But culturally, and in their ethnolinguistic identity, these people are nothing but Scandinavian at this point.
Going back to the results of the 23andMe user above, who genealogically is more than 60% German, but comes back as 25% German, how to make sense of it? Anyone who has looked at German data realizes that it is very difficult to identify a ‘prototypical’ German. Germans are people who speak Germanic languages, whose ancestors out of the European Bronze Age, when much of Northern European population structure was established. But being at the center of Europe means that Germans have been subject to gene flow by peoples to from all other directions. Also, some ethnic Germans in the eastern regions clearly descend from Slavic tribes, and more recently there were migrations of peoples such as French Huguenots.
A PCA of Danes, English, French, and Germans, show differences across the groups. But Germans overlap a great deal with the English, and a substantial minority overlap with Danes. Also, many more of the Germans are “French-shifted” than the English.
The point is that to be German is to be many things. At least in the context of Northern European peoples.