John Caldwell recently posted a message saying his genetic testing revealed similarities to the genetic patterns of inhabitants of Iceland.

I searched the index of the American Journal of Human Genetics onlinem using the keyword “celtic,” and found 23 articles

Most helpful for John are the following articles:

Am. J. Hum. Genet., 67:697-717, 2000
Estimating Scandinavian and Gaelic Ancestry in the Male Settlers of Iceland

Agnar Helgason,1 Sigrún Sigur£ardóttir,3 Jayne Nicholson,2 Bryan Sykes,2 Emmeline W. Hill,4 Daniel G. Bradley,4 Vidar Bosnes,5 Jeffery R. Gulcher,3 Ryk Ward,1 and Kári Stefánsson3

Institutes of [1 ]Biological Anthropology and [2] Molecular Medicine, University of Oxford, Oxford; [3] deCODE Genetics Inc., Reykjavik; [4] Department of Genetics, Trinity College, Dublin; and [5] Blodbanken, Ullevål Sykehus, Oslo

ABSTRACT: We present findings based on a study of Y-chromosome diallelic and microsatellite variation in 181 Icelanders, 233 Scandinavians, and 283 Gaels from Ireland and Scotland. All but one of the Icelandic Y chromosomes belong to haplogroup 1 (41.4%), haplogroup 2 (34.2%), or haplogroup 3 (23.8%). We present phylogenetic networks of Icelandic Y-chromosome variation, using haplotypes constructed from seven diallelic markers and eight microsatellite markers, and we propose two new clades. We also report, for the first time, the phylogenetic context of the microsatellite marker DYS385 in Europe. A comparison of haplotypes based on six diallelic loci and five microsatellite loci indicates that some Icelandic haplogroup-1 chromosomes are likely to have a Gaelic origin, whereas for most Icelandic haplogroup-2 and -3 chromosomes, a Scandinavian origin is probable. The data suggest that 20%–25% of Icelandic founding males had Gaelic ancestry, with the remainder having Norse ancestry. The closer relationship with the Scandinavian Y-chromosome pool is supported by the results of analyses of genetic distances and lineage sharing. These findings contrast with results based on mtDNA data, which indicate closer matrilineal links with populations of the British Isles. This supports the model, put forward by some historians, that the majority of females in the Icelandic founding population had Gaelic ancestry, whereas the majority of males had Scandinavian ancestry.

Am. J. Hum. Genet., 66:999-1016, 2000

mtDNA and the Origin of the Icelanders: Deciphering Signals of Recent Population History

Agnar Helgason,1 Sigrún Sigur©£ardóttir,2 Jeffrey R. Gulcher,2 Ryk Ward,1 and Kári Stefánsson2

[1] Institute of Biological Anthropology, University of Oxford, Oxford; and [2] deCODE Genetics Inc., Reykjavik, Iceland

ABSTRACT: Previous attempts to investigate the origin of the Icelanders have provided estimates of ancestry ranging from a 98% British Isles contribution to an 86% Scandinavian contribution. We generated mitochondrial sequence data for 401 Icelandic individuals and compared these data with >2,500 other European sequences from published sources, to determine the probable origins of women who contributed to Iceland's settlement. Although the mean number of base-pair differences is high in the Icelandic sequences and they are widely distributed in the overall European mtDNA phylogeny, we find a smaller number of distinct mitochondrial lineages, compared with most other European populations. The frequencies of a number of mtDNA lineages in the Icelanders deviate noticeably from those in neighboring populations, suggesting that founder effects and genetic drift may have had a considerable influence on the Icelandic gene pool. This is in accordance with available demographic evidence about Icelandic population history. A comparison with published mtDNA lineages from European populations indicates that, whereas most founding females probably originated from Scandinavia and the British Isles, lesser contributions from other populations may also have taken place. We present a highly resolved phylogenetic network for the Icelandic data, identifying a number of previously unreported mtDNA lineage clusters and providing a detailed depiction of the evolutionary relationships between European mtDNA clusters. Our findings indicate that European populations contain a large number of closely related mitochondrial lineages, many of which have not yet been sampled in the current comparative data set. Consequently, substantial increases in sample sizes that use mtDNA data will be needed to obtain valid estimates of the diverse ancestral mixtures that ultimately gave rise to contemporary populations.