Genetics and eye color: Q & A

Photo by Lincoln Karim
August 3, 2005
Well-fed Fledgling [check out the eye color and also the bulging crop]

Ronnie, a website correspondent, sent in an intriguing question. John Blakeman answers it below. Following that, a series of notes between Blakeman and me on the subject of eye color and genetics.

Marie:

I noticed on a recent photo of the newly fledged tiercel, [the male redail fledgling] the fact that his eyes are hazel, not the traditional gold-brown.

I looked over other photos of Pale Male Jr. and Charlotte, and I think Charlotte's eyes are also the same shade.

Is this unusual?

Maybe you could check with John Blakeman.

Thank you
Ronnie

John Blakeman responds:

I saw this, too, and also wondered about it. The irises of most red-tail immatures are light yellow, sometimes with a greenish cast. But these photos don't show much yellow. They are atypically dark.

How so? I don't know. I don't think it's an aberrant photographic artifact. I've photographed a hundred red-tails, and film accurately depicts such colors. I think the iris colors of these birds are legitimately dark. Genetic? Or a result of food? Almost surely it's genetic. And as with the golden breast color of the young birds, I haven't seen this in my Ohio red-tails. It may be another local color variation.

Was this dark iris color seen in Pale Male Sr's eyasses? If so, it's surely genetic.

One of the many NYC red-tail curiosities.

Subsequently I sent Blakeman a photo of a 2003 Fifth Avenue fledgling with yellow eyes that I found archived on Lincoln's website:

I asked:

Since that's one of Pale Male's offspring, and it has yellow eyes, does this mean the Trump Parc kids are less likely to be grandkids? Is eye color a dominant trait in hawks so that ALL PM's offspring would inherit some sort of darker-eyes-when-juvenile tendency? I seem to have forgotten everything I ever learned in high school about genetics. I just remember something about peas in Mendel's garden...

Here's Blakeman's reply:

Marie,

Mendel struck it rich by happenstance. Purely by luck he picked seven traits in garden peas that just happened to have typical dominance and recessiveness. Biology has been favored by that propitious accident.

But in most traits, things aren't so clear, and I fear that that's the case with the eyass red-tail eye color. It's probably not classically dominant or recessive. Like most traits, it's probably incomplete dominance, or a sort of mixing along a gradient of gene expression that can vary from individual to individual. In short, there is no way of knowing the genetics involved in the eye colors we are seeing. There is no way to know if they are descended from PM Sr. It might even be related to the late (and warm) incubation. As you may know, many reptiles' sex is determined by the temperature at which the eggs are incubated. There is an outside chance this this iris color relates to something having to do with the late incubation period. Like so much, it's all speculation.

Sincerely,

John A. Blakeman