I would challenge you to point out the genetic differences between someone from France and someone from Germany.
By your reasoning there are no Americans since there are no genetic groups that are native to this continent--every group here immigrated from somewhere. By your reasoning everyone in the world is African since every bit of genetic research I've seen traces every so called race to emigrants from Africa.
I do agree with you that the concept of nationhood originated from ethnic/cultural affinity, but the very idea of nation states is a relatively new concept. For most of human history there we no nations.
There's only three genetic groups that would make up three different nations. These genetic groups can be determined by hair structure.
http://www.fbi.gov/hq/lab/fsc/backissu/july2000/deedric1.htm
Caucasoid (European)
Hairs of Caucasoid or Caucasian origin can be of fine to medium coarseness, are generally straight or wavy in appearance, and exhibit colors ranging from blonde to brown to black. The hair shafts of Caucasian hairs vary from round to oval in cross section and have fine to medium-sized, evenly distributed pigment granules.
Mongoloid (Asian)
Hairs of Mongoloid or Asian origin are regularly coarse, straight, and circular in cross section, with a wider diameter than the hairs of the other racial groups. The outer layer of the hair, the cuticle, is usually significantly thicker than the cuticle of Negroid and Caucasian hairs, and the medulla, or central canal, is continuous and wide. The hair shaft, or cortex, of Mongoloid hair contains pigment granules that are generally larger in size than the pigment granules of Caucasian hairs and which often appear to be grouped in patchy areas within the shaft. Mongoloid hair can have a characteristic reddish appearance as a product of its pigment.
Negroid (African)
Hairs of Negroid or African origin are regularly curly or kinky, have a flattened cross section, and can appear curly, wavy, or coiled. Negroid pigment granules are larger than those found in Mongoloid and Caucasian hair and are grouped in clumps of different sizes and shapes. The density of the pigment in the hair shaft may be so great as to make the hair opaque. A Negroid hair shaft exhibits variation or apparent variation in diameter because of its flattened nature and the manner in which it lies on the microscope slide. Twisting of the hair shaft, known as buckling, can be present, and the hair shaft frequently splits along the length.