Blonde hair is traced to a specific genetic mutation on the KITLG gene, which first emerged in the ancient peoples of Siberia and Central Asia roughly 17,000–20,000 years ago. Meanwhile, the gene for blue eyes (OCA2–HERC2 region) appeared much earlier, around 40,000 years ago, likely in a few individuals near the Black Sea region before spreading through Europe.
When these ancient Siberian populations with the blonde hair mutation migrated westward into Europe, they encountered populations already carrying the blue-eye mutation. The combination of these two distinct genetic traits created the phenotype we now associate with Northern Europeans, the “ideal” look that adorns glossy magazines & Hollywood billboards the world over.
The process that created the Sydney Sweeney/Chris Hemsworth phenotype is genetic admixture. Two distinct populations one carrying the Siberian-derived allele for blonde hair and another carrying the West Eurasian-derived allele for blue eyes came into contact, interbred, and created a new admixed population where these previously separate traits could combine in a single individual.
