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B I O T E C H Bas i c

Countless students have

been taught that a single

gene controls eye color, with

the allele for brown eyes

being dominant over blue.

Scientists now realize such

a model is overly simplistic

and incorrect.

Introduction

In 1907, Charles and Gertrude Daven-port developed a model for the geneticsof eye color. They suggested that browneye color is always dominant over blueeye color. This would mean that two blue-eyed parents would always produce blue-eyed children, never ones with browneyes.

For most of the past 100 years, thisversion of eye color genetics has been

taught in classrooms around the world.Its one of the few genetic concepts thatadults often recall from their high schoolor college biology classes. Unfortunately,this model is overly simplistic and incor-rect eye color is actually controlled byseveral genes. Additionally, many of thegenes involved in eye color also inu-ence skin and hair tones. In this editionof Biotech Basics, well explore the sci-ence behind pigmentation and discussthe genetics of eye color. In a future edi-

tion, well discuss genetic factors thatcontribute to skin and hair color.

A primer on pigmentation

The color of human eyes, skin and hairis primarily controlled by the amountand type of a pigment called melanin.Specialized cells known as melanocytesproduce the melanin, storing it in intrac-ellular compartments known as melano-somes. The overall number of melano-cytes is roughly equivalent for all people,however the level of melanin inside eachmelanosome and the number of mel-anosomes inside a melanocyte varies.The total amount of melanin is what de-termines the range of hair, eye and skincolors.

There are a number of genes involvedin the production, processing and trans-port of melanin. Some genes play majorroles while others contribute only slight-

ly. To date, scientists have identie150 different genes that inuencehair and eye pigmentation (an uplist is available at http://www.espcmicemut/). A number of these have been identied from studyinnetic disorders in humans. Othersdiscovered through comparativenomic studies of coat color in micpigmentation patterns in sh. (A ous Biotech101 article that providoverview of comparative genomic

be found at http://www.hudsonalpheducation/outreach/basics/compargenomics.}

Eye color genes

In humans, eye color is determithe amount of light that reects oiris, a muscular structure that cohow much light enters the eye. Thein eye color, from blue to hazel to (see gure one), depends on the le

melanin pigment stored in the msome packets in the melanocytesiris. Blue eyes contain minimal amof pigment within a small number oanosomes. Irises from greenhazeshow moderate pigment levels andanosome number, while brown eythe result of high melanin levels across many melanosomes (see two).

To date, eight genes have beentied which impact eye color. The

gene, located on chromosome 1pears to play a major role in contthe brown/blue color spectrum.produces a protein called P-proteis involved in the formation and proing of melanin. Individuals withmutations that prevent P-proteinbeing produced are born with a foalbinism. These individuals havelight colored hair, eyes and skin.disease-causing OCA2 variants (a

What you need to know:

DNA provides the set of recipes,

or genes, used by cells to carry

out daily functions and interact

with the environment.

Eye color was traditionally

described as a single gene trait,

with brown eyes being dominant

over blue eyes.

Today, scientists have discovered

that at least eight genes inuence

the nal color of eyes. The genes

control the amount of melanin

inside specialized cells of the iris.

One gene, OCA2, controls nearly

three-fourths of the blue-brown

color spectrum. However, other

genes can override the OCA2

instruction, albeit rarely. This

multifactorial model for eye color

explains most of the genetic

factors that inuence eye color.

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THE SCIENCE OF PROGRESS

have also been identied. These allelesalter P-protein levels by controlling theamount of OCA2RNA that is generated.The allele that results in high levels of P-protein is linked to brown eyes. Anotherallele, associated with blue eye color,dramatically reduces the P-protein con-centration.

On the surface, this sounds like thedominant/recessive eye color model that

has been taught in biology classes fordecades. However, while about three-fourths of eye color variation can be ex-plained by genetic changes in and aroundthis gene, OCA2is not the only inuenceon color. A recent study that comparedeye color to OCA2 status showed that62 percent of individuals with two cop-ies of the blue-eyed OCA2allele, as wellas 7.5 percent of the individuals who hadthe brown-eyed OCA2 alleles, had blueeyes. A number of other genes (such as

TYRP1, ASIP and ALC42A5) also func-tion in the melanin pathway and shift thetotal amount of melanin present in theiris. The combined efforts of these genesmay boost melanin levels to produce ha-zel or brown eyes, or reduce total mela-nin resulting in blue eyes. This explainshow two parents with blue eyes can havegreen- or brown-eyed children (an im-possible situation under the Davenport

single gene model) the combination ofcolor alleles received by the child result-ed in a greater amount of melanin thaneither parent individually possessed.

As a side note, while there is a widevariability in eye color, colors other thanbrown only exist among individuals ofEuropean descent. African and Asianpopulations are typically brown-eyed. In

2008 a team of researchers studyiOCA2 gene published results destrating that the allele associatedblue eyes occurred only within th6,000 10,000 years within the Eurpopulation.

Pigmentation research at

HudsonAlpha

Dr. Greg Barsh, a physician-sc

who has recently joined the Hudpha faculty, and his lab study key aof cell signaling and natural variata means to better understand, diaand treat human diseases. In parthis work has focused on pigmendisorders. He has explored mutthat affect easily observable traitsas variation in eye, hair or skin coas a signpost for more complex procsuch as diabetes, obesity, neuroderation and melanoma, the most s

form of skin cancer.- Dr. Neil Lambdirector of educational outreach

Figure onewas modiedfrom SturmRA, Can blue-eyed parentsproduce brown-eyed children?BioscienceExplained,volume 4, issue1 accessedat http://www.bioscience-explained.org

Figure one

Readers may notice the name of this regularly app

educational feature has been changed. Biotech 101,

has occupied the center spread of every Hudson

newsletter, is now Biotech Basics to ease con

between this feature article and HudsonAlphas educa

outreach program for adults, also entitled Biotech 10

Figure two

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