Genetic differences between Britain and Ireland
A key example is provided by the People of the British Isles study, led by Sir Walter Bodmer, which has found rich concentrations of genes of the British Isles' first hunter-gatherer settlers in men and women now living in Cornwall, Devon, Scotland and Ireland. One version of the gene MCR1 often confers red hair on its owners and explains those ancient Roman and Greek reports of widespread ginger locks among early Britons. Red hair was common until invasions by non-redheads - like the Anglo-Saxons - pushed these settlers to Britain's outer edges. Hence the red-haired Scots and Irish we see today.
Bodmer has found signs of Anglo-Saxon genes in east England, the remnants of the invaders who established English as the language of the British Isles, while Wilson's research has discovered evidence that Vikings, who colonised Orkney, did so by eradicating nearly ever male member of its Pictish population. This latter discovery was made by analysing the Y-chromosome. Orkney men today tend to have Y-chromosomes like those of modern Scandinavians, the Vikings' direct descendants.
Relax - we're all Anglo-Saxon anyway!
Myths of British Ancestry
Anglo-Saxons Were Apartheid Racists!
'Apartheid' slashed Celtic genes in early England
We're nearly all Celts under the skin
Y Chromosome Evidence for Anglo-Saxon Mass Migration
English and Welsh are races apart