Human Eye Color Genes [Athro, Limited: Genetics]

Phenotypes and Genotypes for human eye colors

Part of the study of genetics involves being able to predict the appearance (phenotype) and genetics (genotype) of the offspring of animals or people. Sometimes this is just for fun, such as breeding dogs to get a certain color, but sometimes it is very serious, as when dealing with diseases that can be passed from parents to their children. Here is an example of what we know about the genetics of eye color.

There are three known human eye color genes named: 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. EYCL3 (bey2), the Brown/blue eye color gene located on chromosome 15. A role has also been suggested for the Melanocortin Receptor Gene (MCIR) on chromosome 16. A second gene for green has also been postulated. Other eye colors including black, grey, and hazel are not yet explained. We do not yet know what these genes make (except MCIR), or how they produce eye colors. The two gene model (EYCL1 and EYCL3) used here clearly does not explain more than a portion of human eye color inheritance.

Two gene model: There are two genes involved in eye color. One gene has Dominant Brown or recessive blue states, the other has Dominant Green or recessive blue states. We further assume 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. Because chromosomes come in duplicate pairs, each parent has two copies of each gene.

Possible Phenotypes and Genotypes under the two gene model

Phenotype: Blue Eyes

1 Possible Genotype:

Gene: bey2	Gene: gey
Chromosome 15 Copy 1: blue Chromosome 15 Copy 2: blue	Chromosome 19 Copy 1: blue Chromosome 19 Copy 2: blue

Phenotype: Green Eyes

2 Possible Genotypes:

Gene: bey2	Gene: gey
Chromosome 15 Copy 1: blue Chromosome 15 Copy 2: blue	Chromosome 19 Copy 1: Green Chromosome 19 Copy 2: Green
Chromosome 15 Copy 1: blue Chromosome 15 Copy 2: blue	Chromosome 19 Copy 1: Green Chromosome 19 Copy 2: blue

Phenotype: Brown Eyes

6 Possible Genotypes:

Gene: bey2	Gene: gey
Chromosome 15 Copy 1: Brown Chromosome 15 Copy 2: Brown	Chromosome 19 Copy 1: Green Chromosome 19 Copy 2: Green
Chromosome 15 Copy 1: Brown Chromosome 15 Copy 2: Brown	Chromosome 19 Copy 1: Green Chromosome 19 Copy 2: blue
Chromosome 15 Copy 1: Brown Chromosome 15 Copy 2: Brown	Chromosome 19 Copy 1: blue Chromosome 19 Copy 2: blue
Chromosome 15 Copy 1: Brown Chromosome 15 Copy 2: blue	Chromosome 19 Copy 1: Green Chromosome 19 Copy 2: Green
Chromosome 15 Copy 1: Brown Chromosome 15 Copy 2: blue	Chromosome 19 Copy 1: Green Chromosome 19 Copy 2: blue
Chromosome 15 Copy 1: Brown Chromosome 15 Copy 2: blue	Chromosome 19 Copy 1: blue Chromosome 19 Copy 2: blue

Example Crosses:

Shown on a Punnett Square
Example 1
Parent 1: Brown Eyes, Genotype: bey2: Brown-blue, gey:blue-blue
Parent 2: Blue Eyes, Genotype: bey2: blue-blue, gey:blue-blue

bey2: gey:	Brown blue	Brown blue	blue blue	blue blue
blue blue	Brown-blue blue-blue	Brown-blue blue-blue	blue-blue blue-blue	blue-blue blue-blue
blue blue	Brown-blue blue-blue	Brown-blue blue-blue	blue-blue blue-blue	blue-blue blue-blue
blue blue	Brown-blue blue-blue	Brown-blue blue-blue	blue-blue blue-blue	blue-blue blue-blue
blue blue	Brown-blue blue-blue	Brown-blue blue-blue	blue-blue blue-blue	blue-blue blue-blue

Each child has 50% probablility of brown eyes and 50% probability of blue eyes. On average, about half the children of this cross will have brown eyes and the other will have blue eyes

Example 2
Parent 1: Brown Eyes, Genotype: bey2: Brown-blue, gey:blue-blue
Parent 2: Green Eyes, Genotype: bey2: blue-blue, gey:Green-blue

bey2: gey:	Brown blue	Brown blue	blue blue	blue blue
blue Green	Brown-blue Green-blue	Brown-blue Green-blue	blue-blue Green-blue	blue-blue Green-blue
blue Green	Brown-blue Green-blue	Brown-blue Green-blue	blue-blue Green-blue	blue-blue Green-blue
blue blue	Brown-blue blue-blue	Brown-blue blue-blue	blue-blue blue-blue	blue-blue blue-blue
blue blue	Brown-blue blue-blue	Brown-blue blue-blue	blue-blue blue-blue	blue-blue blue-blue

Example 3
Parent 1: Brown Eyes, Genotype: bey2: Brown-blue, gey:Green-blue
Parent 2: blue Eyes, Genotype: bey2: blue-blue, gey:blue-blue

bey2: gey:	Brown blue	Brown Green	blue Green	blue blue
blue blue	Brown-blue blue-blue	Brown-blue Green-blue	blue-blue Green-blue	blue-blue blue-blue
blue blue	Brown-blue blue-blue	Brown-blue Green-blue	blue-blue Green-blue	blue-blue blue-blue
blue blue	Brown-blue blue-blue	Brown-blue Green-blue	blue-blue Green-blue	blue-blue blue-blue
blue blue	Brown-blue blue-blue	Brown-blue Green-blue	blue-blue Green-blue	blue-blue blue-blue

Each child has 50% probablility of brown eyes and 25% probability of green eyes and 25% probablity of blue eyes. On average, about half the children of this cross will have brown eyes, one quarter green eyes, and one quarter blue eyes. About half of the children with brown eyes will carry the Green allele for the gey gene, and the other half will carry the blue allele.

Example 4
Parent 1: Brown Eyes, Genotype: bey2: Brown-blue, gey:Green-blue
Parent 2: Brown Eyes, Genotype: bey2: Brown-blue, gey:Green-blue

bey2: gey:	Brown blue	Brown Green	blue Green	blue blue
Brown blue	Brown-Brown blue-blue	Brown-Brown Green-blue	blue-Brown Green-blue	blue-Brown blue-blue
Brown Green	Brown-Brown blue-Green	Brown-Brown Green-Green	blue-Brown Green-Green	blue-Brown blue-Green
blue Green	Brown-blue blue-Green	Brown-blue Green-Green	blue-blue Green-Green	blue-blue blue-Green
blue blue	Brown-blue blue-blue	Brown-blue Green-blue	blue-blue Green-blue	blue-blue blue-blue

Each child has a 75% probablility of brown eyes and a 18.75% probability of green eyes and a 6.25% probablity of blue eyes. On average, about three quarters of the children of this cross will have brown eyes, three sixteenths green eyes, and one sixteenth blue eyes.

You can explore this further with an interactive eye color calculator

Sources:Eiberg and Mohr, 1996; Eiberg and Mohr, 1987;
Online Mendelian Inheritance in Man, OMIM (TM). Johns Hopkins University, Baltimore, MD. MIM Number: 227220: 6/15/1999: (EYCL3). World Wide Web URL: http://www.ncbi.nlm.nih.gov/omim/
Online Mendelian Inheritance in Man, OMIM (TM). Johns Hopkins University, Baltimore, MD. MIM Number: 227240: 7/21/1998 : (EYCL1) . World Wide Web URL: http://www.ncbi.nlm.nih.gov/omim/

Part of the Athro, Limited web site.
Copyright © 1999 Athro, Limited. All Rights Reserved.
Written by Paul J. Morris mole@morris.net
Maintained by Athro Limited
Date Created: 11 Dec 1999
Last Updated: 11 Dec 1999