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Inherited Disorders and Birth Defects

How to determine eye color

02/11/2002

Question:

to determine a person having hazel eyes or grey and blue eyes what is the genetic component? does that mean both of the genes are present and they clash?

Answer:

The following comes from a terrific website listed below that explains what is known about the genetics of eye color. The website is done by Athro Limited.

First, eye color is complex. In humans, there are three genes known to be involved in eye color. They explain typical patterns of inheritance of brown, green, and blue eye colors. The 3 eye color genes that are known are: EYCL1 (gey), the Green/blue eye color gene, located on chromosome 19; EYCL2 (bey1), the central brown eye color gene, possibly located on chromosome 15; and EYCL3 (bey2), the Brown/blue eye color gene located on chromosome 15. We do not know what these genes make - what proteins they produce, or how the proteins produce eye colors. Eye color at birth is often blue and later turns to a darker color. Why eye color can change over time is not known.

Researchers also think there is a second gene for green as well. However, these genes don`t explain everything. Gray eye color, Hazel eye color, and multiple shades of blue, brown, green, and gray are not explained.

It is thought that for brown, blue and green there is a two-gene model; that is, there are two genes involved in eye color. The model says that because chromosomes come in duplicate pairs, each parent has two copies of each gene. One gene has Dominant Brown or recessive blue states, the other has Dominant Green or recessive blue states. It is assumed that Brown in the first gene will swamp out Green or blue from the second gene, and Green from the second gene will swamp out blue from the first gene. This gives a hierarchy of one gene being dominant over another - thus for the 3 known genes for eye color, bey2-brown is dominant over everything else, gey-green is dominant over bey2-blue and gey-blue but recessive to bey2-brown, and both of the blues are recessive to everything else.

Of course, this two gene model (EYCL1 and EYCL3) clearly does not explain more than a portion of human eye color inheritance. Additional eye color genes and modifier genes must also be involved.

Go to the website below for more explanation. There are interactive areas to try different eye colors and what they might produce.

Related Resources:

Inheriting Eye Color by Athro Limited

For more information:

Go to the Inherited Disorders and Birth Defects health topic, where you can:

Response by:

Anne   Matthews, RN, PhD Anne Matthews, RN, PhD
Associate Professor of Genetics
School of Medicine
Case Western Reserve University