Inheritance of eye color doesn’t follow a simple
Mendelian model. Although the blue-eye allele (C) is less dominant than the
brown-eye allele (T), CT heterozygotes aren’t necessarily brown-eyed and CC
homozygotes aren’t necessarily blue-eyed. Even TT homozygotes are sometimes
blue-eyed. There is also a sex difference, with women having a more diverse palette
of eye colors. (source)
Most humans have black hair and brown eyes.
Europeans display a much more diverse range of hues, their hair being also
brown, flaxen, golden, or red and their eyes being also blue, gray, hazel, or
green.
This diversification has gone farther in European
women than in European men. According to a twin study, women have a more
diverse palette of hair colors, with a greater prevalence of lighter shades,
particularly red hair (Shekar et al., 2008).
Women also have a more diverse palette of eye colors,
according to a recent study of six SNPs associated with eye color. When both copies
of the so-called blue-eye allele are present, the resulting phenotype is more
variable in women than in men (Martinez-Cadenas et al., 2013). This translates
into a greater range of female eye colors in regions, like northern and eastern
Europe, where blue eyes are the single most common phenotype (Walsh et al.,2012). As the study’s authors observe, “in populations with very high blue-eye
frequency, such as Iceland or Holland, females show greater proportion of green
eyes at the expense of blue eyes” (Martinez-Cadenas et al., 2013). The authors
also confirmed earlier findings that eye color doesn’t follow a simple
Mendelian model. Although the blue-eye allele (C) is less dominant than the
brown-eye allele (T), CT heterozygotes aren’t necessarily brown-eyed and CC
homozygotes aren’t necessarily blue-eyed. Even TT homozygotes are sometimes
blue-eyed.
Thus, both hair color and eye color tend to be more
diverse in women. There is, however, one difference. With hair color, the “derived”
alleles are more fully expressed in women than in men. With eye color, they are
less fully expressed. This seems to argue against the theory that hair and eye
color diversified through a process of sexual selection that acted more
strongly on women than on men. Since blue eyes are the derived phenotype, they
should be more fully expressed in women because the female sex is the main
target of this selection pressure. Yet the reverse is actually true.
The reason may be physiological. It seems easier to
produce new eye colors by modifying the way the blue-eye allele is expressed
than by simply creating new alleles. Thus, to produce a range of hues that
extends beyond brown and blue, the so-called blue-eye genotype must be more
common than the actual incidence of blue eyes. As a result, “more females
bearing the ‘blue eye genotype’ (HERC2/OCA2 CC homozygous genotype) end up
having green or intermediate eyes” (Martinez-Cadenas et al., 2013).
Other recent
studies
A recent paper has confirmed that European eye color
diversified through some kind of selection pressure, and not random factors
like genetic drift or founder effects. Blue-eye alleles show a very strong
signal of selection (Donnelly et al., 2012). Another study, however, has failed
to find any preference for blue eyes over other colors, an indication that all
eye colors are at selective equilibrium, at least for the German population under
study. This finding may be related to the already high frequency of blue eyes in
that population:
Perhaps the frequency of eye
colors plays a role. In most countries, blue eyes are less prevalent than other
eye colors and may have the image of something special and more valuable. If
this assumption is true, brown eyes should be preferred in countries where the
majority of the population has blue eyes. (Gründl et al, 2012).
In this case, sexual selection is
frequency-dependent, shifting to whichever eye color is least frequent. Eventually,
an equilibrium is reached where color novelty is in balance with other
characteristics, such as color brightness, that may increase sexual
attractiveness.
This last finding shows the opportunistic nature of
sexual selection. When too many of one sex have to compete for mating
opportunities with too few of the other sex, there will be selection for any
traits that increase mating success. In many cases, these traits will hyperstimulate
a mental algorithm that is used for sex recognition. In other cases,
hyperstimulation will simply involve use of bright or novel colors that can
better engage visual attention and remain longer in memory.
References
Donnelly, M.P., P. Paschou, E. Grigorenko, D. Gurwitz, C. Barta, R-B. Lu,
O.V. Zhukova, J.-J. Kim, M. Siniscalco, M. New, H. Li, S.L.B. Kajuna, V.G.
Manolopoulos, W.C. Speed, A.J. Pakstis, J.R. Kidd, and K.K. Kidd. (2012). A
global view of the OCA2-HERC2 region and pigmentation, Human Genetics, 131,
683–696.
http://europepmc.org/articles/PMC3325407
Gründl, M., S. Knoll, M. Eisenmann-Klein, and L.
Prantl. (2012). The blue-eyes stereotype: Do eye color, pupil diameter, and
scleral color affect attractiveness? Aesthetic
Plastic Surgery, 36, 234–240.
Martinez-Cadenas, C., M. Pena-Chilet, M.
Ibarrola-Villava, and G. Ribas. (2013). Gender is a major factor explaining
discrepancies in eye colour prediction based on HERC2/OCA2 genotype and the
IrisPlex model, Forensic Science
International: Genetics, 7,
453–460.
Shekar, S.N., D.L. Duffy, T. Frudakis, G.W.
Montgomery, M.R. James, R.A. Sturm, & N.G. Martin. (2008).
Spectrophotometric methods for quantifying pigmentation in human hair—Influence
of MC1R genotype and environment, Photochemistry
and Photobiology, 84, 719–726.
Walsh, S., A. Wollstein, F. Liu, U. Chakravarthy, M.
Rahu, J.H. Seland, G. Soubrane, L. Tomazzoli, F. Topouzis, J.R. Vingerling, J.
Vioque, A.E. Fletcher, K.N. Ballantyne, and M. Kayser. (2012). DNA-based eye
colour prediction across Europe with the IrisPlex system, Forensic Science International: Genetics, 6, 330–340.
http://www.fsigenetics.com/article/S1872-4973(11)00144-X/abstract
