In the course of teaching, one good sign that arises in various course designs is the comfort and curiosity that leads people to ask certain questions. Meaning, the kinds of questions that they are pretty sure the course subject avoids or denies or is simply wrong about, or the ones that have always led them privately to wonder “how” with little faith that anyone could or would answer.
A nearly guaranteed topic for these is human eye color. How is it, they ask, that if eye color is “genetic,” that people can have eyes of different colors? Or that change through the course of life? Or aren’t “brown” exactly? Or … (the list goes on). Crucially: the student asks in such a way as to indicate challenge, by which I don’t necessarily mean hostility, but rather an underlying assumption that this feature is observationally different from what “biology says.” Eye color defies what they think genes are supposed to be doing.
This is an important issue in the interfaces among education, popular understanding or lack thereof, and actual biology, but to address it, let’s first take a look at mammalian, specifically human eye anatomy and composition.
- There’s the surface of the iris: smooth or rough. A smooth iris transmits light; a rough one reflects it, and as it happens, the particular sort of roughness we’re talking about reflects blue.
- There are the pigments: pheomelanin (red) and eumelanin (brown). Either or both can be present.
Eye color is the result of any degree of surface roughness with any degree of either pigment – wait, quick amendment to that: apparently, pheomelanin has an upper limit in density for eyes such that we don’t see red-pigmented irises, so think of it as various degrees of yellow or gold). But for example, a light density of pheomelanin plus a rough surface obviously reflects both yellow and blue, respectively, for the combined effect of green. Unsurprisingly when we are talking about combining yellow, blue, and varying degrees of brown, the possible nuances of “brown” are legion. Hence honey, amber, hazel, black, et cetera et cetera, with no Hollywood lenses necessary.
If one has smooth-surfaced irises with no pigment at all, they are transparent, so the blood vessels behind the pigment reflect red – this is why albinos have either blue eyes, due to the surface of the iris, or pink-to-red eyes, which are not otherwise observed in humans.
So … how about inheritance? How many genes? Yeesh. There’ve gotta be dozens, and probably several in sequence for any particular effect. Let’s dispose of one problem immediately: the whole blue/brown, dominant/recessive thing for eyes is simply arrant nonsense. I don’t know why or how it got started as standard pedagogy, but it ranks high in the annals of flat-out lying to students. Simplification is one thing; falsehood is another.
In reality, eye color is just like many other biological variables with multiple genetic influences: its inheritance is consistent enough to start thinking it’s fixed, until it isn’t. It’s a good example of how human experience (what you see, what you “know”) is statistically innumerate Yet again, high significance + low correlation coefficient – I should post more about that some time.
I’ll also hold off on discussing eye pigmentation vs. skin pigmentation, and for that matter, hair – the only thing to say for now is that all three show geography-specific variation, but also that there’s no physiologically necessary association among the developmental processes of each, and if there are ties among those processes, they are multi-variate and not absolute.
- Different in different lighting: Obviously. This is such simple physics it bears no further comment.
- Different via association/contrast: Also not surprising, although the mechanisms are more interesting – the perceiving brain may seek matches among fields it strongly associates (i.e. located on the same person) and adjust the input accordingly.
- Different left/right: Yup. Slight asymmetry of all sorts of left-right bodily features is the norm, not the exception, and occasional past-norm instances are common. (No, David Bowie did not have one brown and one blue eye; he had a non-color-related condition in which his pupils were of differing sizes.)
- Different colors for eyes as one matures: Again, so common as to be the norm, especially for pigmentation. My hair was white-blond as a little kid, quite gold through my teens and twenties, and straightforward brown ever since.
- Different fields on the iris, sometimes “wedges” or flecks but very commonly central heterochromia, for a “ring” or “star” effect surrounding the pupil. Sometimes the result for the viewer is a gestalt, as their brain constructs a color which isn’t actually produced by the eye, and sometimes the two are perceived as such.
For instance, my eyes are darkish (“steel”) blue but with a lighter, essentially white field immediately surrounding the pupil. Both of my brothers and all three of my kids have the same eyes, as do a number of relatives on our father’s side. At the risk of being dramatic, people don’t like them very much; clearly the ordinary attention that people pay toward the direction of others’ gazes (which is easy to see in humans anyway, due to our small irises) is triggered by the extra visibility of the pupil. It takes very little actual looking before someone gets offended at “staring” or the “way” they feel they’re being looked at. I wore thick glasses as a kid and switched to contacts as soon as I could, but a few years later, I went back to glasses (fortunately tech had now made thinner lenses possible), because I was tired of people responding to casual eye contact with genuine primate threat-displays, exactly as if I’d just done the same to them.
You see what’s revealed by the questions, right? Even putting the nonsense about Mendelian blue/brown aside, somehow, people have been getting the idea that because eye color is “genetic,” it must be fixed in inheritance pattern, immutable per person, invariant across people, and essentially tied to that person’s identity. This isn’t about eyes but about what people think the word “genes” means. It’s tied to the profound failure to teach (not to understand, to teach) that actual bodily features come from development and physiology, not genes. Genes are the inheritable component thereof, not the “core” or “essence” thereof.
I call special attention to what any bio teacher grapples with, that ordinary critical thinking is short-circuited – no one thinks it odd that hair color is an inheritable feature and also undergoes personal changes, but somehow for eye color to do so is jarring and “non-genetic.”
I categorized this post as “Talking it over” for a reason. The crude and counter-factual thinking is so deeply embedded that it’s impossible for people to state – why should they or how can they, state “the obvious?” It’s only revealed as assumptions behind questions. Ask me questions about genes, specifically for things which seem grossly physically obvious. Let’s get into this right here in the comments.
Links: The Tech
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