Dear Matthew Alice:
Now that my father has passed away, I "inherited" boxes that filled a basement, attic, and two-car garage (into which my father never put a car.) Amidst the papers, pictures, and assorted historic items there was a family tree going back into the late 17th Century. In a rare spare moment, I noted eye colour[CQ] beside each member. Virtually every male and female had blue eyes, except for me. Mine are hazel, though my mother assured me they were blue on my birth day[CQ two words] but "changed" sometime in the first year. My wife and all of her family also have blue eyes, as do our three children and all of our grandchildren. Recalling genetics taught in school, I thought that blue eye colour is a recessive gene, not allowing for other eye hues of descendants. Is it possible for me to have hazel eyes under those extraordinary circumstances? Have I discovered a family secret?
-- Buried in Boxes
If you're old enough to have grandchildren, be assured that everything you learned in biology class is now wrong. Time and science march on. One of the "facts" jettisoned by contemporary research is the assumption that only two genes predict eye color. Science guys are pretty sure there are genes they haven't even found yet that play some part in each person's ultimate peeper pigmentation. Though it might be an unusual occurrence, you (and millions of other kids) are testimony to the fact that two blue-eyed parents (or five blue-eyed generations) can produce a non-blue-eyed child. If we'd known this sooner, perhaps the divorce rate would be lower.
The main source of eye color is a layer of cells in the iris that contains deposits of pigment. The color we see is created by light entering the eye and being scattered by this layer. The size, shape, and density of these layered cells, plus the size and intensity of the pigment spots and their distribution within the cells determine the scatter pattern, and thus eye color and color depth. (Why is the sky blue? Same reason eyes are blue-- conditions that scatter blue light.) Gene patterns will influence the characteristics of this iris cell layer. How many genes? What's their influence? Stay tuned. Nobody's sure yet.
So far, eye science has identified three genes that are eye-pigment linked: green-blue, brown-blue, and brown. These will influence the amount of pigment (melanin) in the iris; at one extreme, dense deposits of melanin help make eyes look some shade of what we'd label "brown." At the other end of the continuum, minimal deposits create shades of "blue." And given a polygenic model for eye color, a family with predominantly blue-eyed ancestors can still carry a genetic potential for a brown- or hazel-eyed child. Actually, greenish eyes are colored by an additional pigment, lipochrome, so there's another viariable in the chromatic mix. Aren't you glad you took biology back when life was simpler?