Sex tweaks eye color

Abstract Here I challenge the conventional view of eye-color inheritance by showing that eye color depends (doubly) on sex: the sex of the parent and the sex of the child. I rely on the eye color records of over 30,000 Italians—a single-ethnicity population that features the largest genetic and eye-color diversity in Europe. First, more men than women express eye colors at the two extremes of the melanin range (blue and dark-brown) and more women than men express colors in the middle (green, hazel, brown, and mixed). Second, dark-brown-eyed mothers produce more sons with dark-brown eyes and fewer with very-light-brown ones than do dark-brown-eyed fathers. Symmetrically, dark-brown-eyed fathers produce more daughters with dark-brown eyes and fewer with very-light-brown ones than do dark-brown-eyed mothers. All of this suggests that human eye pigmentation is controlled by a gene sitting on chromosome X—a chromosome which fathers pass on only to daughters, and sons inherit only from mothers. Such knowledge can improve forensic tools’ ability to predict eye color from DNA found at crime scenes and help in the identification of a known individual’s unknown birth parents. On a grander scale, these findings cast a new light on the origin of other, more fateful sex differences.