In Europe, PC analysis has revealed the strongest genetic differentiation between the northwest and southeast of the continent. The first PC accounts for approximately twice as much of the genetic variation as PC2 [12], [13], [15]. In addition, Price et al. (2008) have shown in their study of US Americans of European descent that the consideration of three clusters of individuals, which roughly corresponded to Northwest Europe, Southeast Europe and Ashkenazi Jewish ancestry, may be sufficient to correct for most of the population stratification affecting genetic association studies. However, the extent to which the results of PC analysis reflect the true underlying genetic map of Europe is critically dependent upon the choice of populations analyzed. Optimal coverage of European populations has not been achieved so far and still represents a goal for future collaborative studies. At present, however, it appears essential that the peripheral populations of Europe or those with a strong founder effect in particular must not be left out of studies aiming at the construction of a continent-wide genetic map.
