Atlantic modal haplotype (AMH).

Haplotype 1.15 is also modal in the Basques and constitutes 41% of the sample, rising to 56% for the cluster of one-step neighbors. This is called the Atlantic modal haplotype (AMH). In each of the Basque, Welsh, and Irish populations, a total of 89 to 90% of the chromosomes are in hg 1, which contains the M173-defined Eu18 hg, with the majority of the remainder in hg 2. The AMH and one-step neighbors are present in the target populations but only one chromosome from this group is found in the Near Eastern samples, and it is absent in India and Central Asia samples. There is no evidence, therefore, that incoming Neolithics or later immigrants originating in the Near East carried the AMH at frequencies as high as those characterizing the Atlantic populations.

Studies suggest the possibility of a Basque/Celtic connection since they show Irish and Basque populations falling very near one another on the first principal component axis, which reflects the spread of Neolithic farmers from the Near East. The relative proximity of the Basque and Irish on this axis reflects the relatively small Neolithic component in these populations. More recently studies have used a northwest to southeast cline through Europe of p49a, f haplotype XV [which forms a subclade of hg 1] to argue that hg 1 in Ireland must be old. All pairwise comparisons of hg distributions between the European and Near Eastern populations are significantly different except for those within the Atlantic group—Welsh, Irish, and Basques—none of which are distinguishable, showing that they form a Y-chromosome community with members more closely related to one another than they are to the other European populations. It should be noted that Basque-Celtic similarity not only implies that Basque- and Celtic-speaking populations derive from common paternal ancestors, but that genetic drift in these communities has not been sufficiently great to differentiate them.

The signal of Basque-Celtic similarity depends to a large degree on the AMH, which has much higher frequency in these populations than in other European populations. With one-step neighbors, the AMH composes only 38% of the Frisian sample (significantly different, P 0.05), consistent with the view that the Basques are genetically distinguishable from continental populations generally. As three alleles within this six-locus haplotype are known to follow a southeast to northwest cline in Europe, it is likely that most other European populations will have even lower frequencies than the Frisians. Both the Basque and the Celtic populations show high frequencies of the AMH. Because the former are generally considered to have received a very limited input of Near Eastern genes in the Neolithic, that similarity also suggests that in the British Isles the Neolithic transition did not entail a major demographic shift. Accordingly, farming may have spread in Britain more through cultural transmission than through migration.