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Genetics and Eye Color
Stephanie Mark
Salt Lake Community College
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A little over 100 years ago, children were taught about genetics through a diagram
created by Charles and Gertrude Davenport (Sturm 2008). They used eye color as the basis to
teach Mendel’s principle of independent assortment to demonstrate dominant (brown eyes) and
recessive (blue eyes) traits and inheritance. Due to the prevailing belief at that time, eye color
was assumed to be determined only by a single gene (Jurmain et al, 2013; Lamb, 2013). Later it
was found that two blue-eyed parents could produce a brown-eyed child (Sturm, 2008). Also,
the simplicity of the Davenport model could not explain the child with hazel or green eyes, the
very rare violet and red eyes, or heterochromia iridium– having one blue eye and one brown eye.
Eye color and its genetics are now determined to be more complex than the simple diagram of
dominant and recessive traits originally designed by the Davenports in 1907 to explain
Mendelian genetics. Eye color is due to the action and interactions of many genes and described
as a polygenic trait (Jurmain et al, 2013). The iris of the eye is as individual as a fingerprint.
Thus, the purpose of this paper is to explore the factors affecting eye color, evolutionary
changes, and the implications of future research of eye color.
Predicting eye color has always been interesting to teachers and students, but
unfortunately not to research geneticists (Frudakis et al, 2007). Many people have tried to use a
simple Punnett square to predict the possibility of having a child with a preferred phenotypic
expression of their assumed genotypes. Research geneticists have also tried to make predictions
on the phenotypic expression of eye color based on the genotypes of participants but they have
determined that the inheritance of eye color is extremely complex and unable to be predicted
with accuracy (Frudakis et al, 2007). Evidence by research in the last 20 years supports that eye
color is caused by a “quasi-Mendelian” inheritance (Frudakis et al, 2007, pg. 312)- that is
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phenotypic expression of eye color is still controlled by dominance but the process of epistasis,
gene suppressors, modifying genes and penetration also affect phenotypic expression or the
actual eye color each individual has. It is now known that there are at least 16 genes that affect
eye color (White & Rabago-Smith, 2011) with multi-loci on the genes that are involved in
affecting eye color. These same genes also affect hair and skin color (Donnelly et al, 2011).
DNA and gene mapping of eye color has been attempted but the myriad of possibilities and
interactions between the genes have made this task daunting for researchers to identify which
genes cause every eye color including the rich variety of hues and variations noted (Frudakis et
al, 2007; White & Rabago-Smith, 2011). Therefore, at this time it is not possible to precisely
predict eye color from the genotype of a person because phenotypic expression is controlled by
so many factors that can even change over time (White & Rabago-Smith, 2011).
White & Rabago-Smith (2011) state that two genes (HERC2 and OCA2) on chromosome
15 are credited with the greatest role in the phenotypic expression of eye color. Epistasis is the
interaction of two alleles that causes phenotypic expression (McClean, 2000). It is now known
that the interaction of these two genes- HERC2 and OCA2- will cause either brown or blue eye
color as well as varying shades of each color. Simply, their interaction affects the quantity and
quality of melanin in the cytoplasm and iris of the eye which then determines the perceived eye
color. Large amounts of melanin causes brown eye color and little melanin produces a blue color
(White & Rabago-Smith, 2011). But the iris has more than one layer with melanin pigment and
melanin pigment has two forms- Eumelanin and Pheomelanin (Frudakis et al, 2007; Sturm,
2008). All eyes appear to have the same number of melanocytes (melanin producing cells), but
not the same amount of melanin produced. Melanin absorbs light that hits it and creates the
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appearance of darkness; the hue or shade of the darkness is caused by the amount of melanin in
the layers of the iris. Depending on the type of melanin produced also will determine the eye
color observed (Sturm, 2008). There are complicated chemical processes involved in producing
melanin that are not completely understood. It is believed that the distribution of the melanin and
the structures of the iris are affected by the amount and type of light. Lighting and its
complexities will also cause an observer to note eye color variations on one individual depending
on the type, quantity, and quality of lighting due to the type of wavelengths hitting the melanin
(Sturm, 2008). Thus, a person can state that others perceive their eye color as having multiple
expressions (hazel eyes can appear brown or green) and be correct all due to lighting and its
wavelengths.
From an evolutionary standpoint, brown eye color is believed to be the oldest eye color
with the blue eye color arising approximately 6,000-10,000 years ago (Cunningham, 2008;
Lamb, 2013). Frudakis et al, (2007) states that most blue-eyed people have the same genotype.
The research indicates that the mutation noted in the OCA2 gene points to a common ancestor
for all blue-eyed persons who is believed to have originated in the Black Sea region of Europe
and then migrated to northern Europe in the Neolithic period (Eiberg et al, 2007). It is proposed
that the predominance of blue eyes were furthered by sexual selection due to them being
preferred over brown eyes leading to the prevalence of blue eyes noted today in northern Europe
(Eiberg et al, 2007). Other reasons for the evolutionary change could be due to less sunlight in
northern Europe causing a Vitamin D deficiency and skin cancers (Eiberg et al, 2007). Donnelly
et al (2011) genotyped approximately 3,400 people from 72 different populations world-wide
and determined that mutations in the OCA2 gene are responsible for blue eyes and are
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appreciably the same in all who have blue eyes. This study gave further credit to the prevailing
belief that all blue-eyed individuals world-wide arise from one common ancestor.
In conclusion, eye color is determined by multiple genes and factors such as lighting and
age. It arises from a complex process of gene interaction and can also change over time. Blueeyes are fairly new on the evolutionary chart with the common ancestor only being 6,000-10,000
years old. Since the genes that control eye color also affect hair and skin color and have been
found in human genetic diseases such as Prader-Willis and Angelman’s syndrome, albinism,
melanoma and others (White & Rabago-Smith, 2011), there is research to determine how the
genes interact and are involved in these specific diseases and potentially how to prevent it.
Further research of eye color is focusing on causes of different hues and saturations in order to
have greater understanding of factors that determine eye color (White & Rabago-Smith, 2011).
Another implication for enhanced genotypical mapping of eye color with predictable
phenotypical expression is for forensics (White & Rabago-Smith, 2011). DNA from crime
scenes could be analyzed and potential suspects could be narrowed down to a specific eye color
thus eliminating all other suspects. Parents could impact the eye color of their offspring before
birth. Eye color could be changed by taking a medication that impedes melanin production or
enhances it. The implications for future research are limitless with the advances made in
technology and science. It will be interesting in the future to see what new discoveries about eye
color arise and how this will affect our understanding of how we see each other.
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References
Cunningham, K. (2008) A Single DNA difference in the HERC2 Gene Explains Blue Eyes.
Retrieved Novemeber 26, 2013 from http://genetics.thetech.org/original_news/news76
Donnelly, M. P., Paschou, P., Grigorenko, E., Gurwitz, D., Barta, C., Lu, R. B., Zhukova, O.V.,
Kim, J-J., Siniscalso, M., New, M., Li, H., Kajuna, S.L.B., Namolopoulos, V.G., Speed,
W.C., Pakstis, A.J., Kiss, J.R., & Kidd, K. K. (2012). A global view of the OCA2HERC2 region and pigmentation. Human genetics, 131(5), 683-696.
Eiberg, H., Troelsen, J., Nielsen, M., Mikkelsen, A., Mengel-From, J., Kjaer, K.W., & Hansen,
L. (2008). Blue eye color in humans may be caused by a perfectly associated founder
mutation in a regulatory element located within the HERC2 gene inhibiting OCA2
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Frudakis, T., Terravainen, T.,& Thomas, M. (2007). Multilocus OCA2 genotypes human iris
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Lamb, N. Biotech Basics: Genetics of Eye Color. Retrieved November 26, 2013 from
http://www.hudsonalpha.org/sites/default/files/pdf/genetics_of_eye_color.pdf
McClean, P. (2000). Mendelian Genetics. Retrieved November 26, 2013 from
http://www.ndsu.edu/pubweb/~mcclean/plsc431/mendel/mendel6.htm
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White, D., & Rabago-Smith, M. (2010). Genotype–phenotype associations and human eye color.
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