A Beginners Guide to Genetics: Eye Color

The Basics

Genetics is a fascinating field of study, and one hub is by no means going to cover even a fraction of the knowledge, work and debate on the subject.

However, most of us will have some curiosity about how we got Grandpa's nose or Aunt Mary's temper, and this hub looks at the heredity of eye color in particular.

For the sake of simplicity, I have assumed there are only two shades of eye colour, Blue and Brown. There are, obviously, many different hues, but the principle is best illustrated using two differentials, with the assumption that any other eye color (green, hazel etc) is just a variant of either Blue or Brown eyes.

DNA and Heredity

On a simple level our DNA (Deoxyribonucleic Acid) is the recipe for how we are made. When we were simple fertilised embryos, this recipe was used to tell the cells how to grow and form and determines many factors, such as height, intelligence, sex and, or course, eye color.

Our genes are made up of two alleles that hold genetic information. We each receive one allele from each parent to make up our own individual set of two. The combined instructions in both alleles will determine the end result of whichever genetic trait is under examination.

For our illustration, the allele for Brown eyes will be BR and the allele for Blue Eyes will be bl. The capitalization is intended, and is explained below.

photo by Matteo 86 from www.sxc.hu
photo by Matteo 86 from www.sxc.hu

Dominant and Recessive Genes

So, each person has two alleles to determine eye color. Its is relatively simple to deduce that a person whose alleles are BR BR will have brown eyes, and that someone who has bl bl alleles will have blue eyes. But what happens with mixed alleles?

In many circumstances, one type of allele is dominant and another is recessive. For eye color, the brown eye allele is dominant BR (hence the capitals) and the blue eye allele is recessive bl. This means that someone who has alleles of BR bl or bl BR will necessarily have brown eyes, as the dominant trait takes precedence.

So what does this mean?

Well, consider the case of a test subject. We'll call him Ira. Ira has brown eyes. Ira's father also has brown eyes, but his mother has blue eyes. The potential genetic make up of Ira's family's alleles are as follows:

Because Ira's mother has blue eyes, we can be sure her alleles are both bl as this is the recessive gene.

Ira's father must have one BR allele, but the second could be either BR or bl.

Ira could potentially have the alleles bl BR or bl bl, depending on his father's second gene. However, as Ira has brown eyes, his alleles must be bl BR.

Genetic Detectives

For eye color, whether Ira's father has two BR genes or BR bl genes is possibly interesting, but not necessarily important. However, for some genetic diseases or abnormalities, it can be important to determine exactly what the combination of genes is.

In most cases, DNA typing can easily solve the mystery. However, in our simplified example, some circumstancial evidence can help us solve the mystery of Ira's father's genes.

If Ira has a sibling with blue eyes, that sibling must have the alleles bl bl, meaning Ira's father must also have passed a bl gene to that child, making his genes BR bl.

So what about Ira's Children?

Because Ira has mixed alleles, BR bl, no matter what color eyes his wife has, his children have the potential to have either blue or brown eyes. If Ira's brown eyes were BR BR, no matter what color his wife's eyes are, his dominant BR gene would mean all his children would have brown eyes. Similarly, two blue eyed parents cannot produce a brown eyed child, as there would be no dominant BR gene to pass on.

Comments 17 comments

8 years ago

This doesn't explain green eyes or the many different variations. I think it will be proven to be simplistic and unaccurate.


8 years ago

Agree with the above comment. I also know a couple where the father is black and the mother is white (also blonde and blue-eyed). Their daughter is blue-eyed, although, by this theory, she couldn't be, as her black father would have only the "BR BR" gene due to his race.


Maureen 8 years ago

My mother and father both have blue eyes. My brother and sister have brown eyes. Is this possible in any way? Say grandparents carried the BR bl gene or maybe one of my parents was BR bl. Is is possible for them to have brown eyed children or is there something they are not telling us?


Bizzit 7 years ago

This was helpful :)


CLOUD DUNN 6 years ago

MY EYES ARE BLUE WITH NO OTHER COLORS MIXED IN ANYWHERE. MY MOTHER HAS GREY EYES. MY FATHER HAS HAZEL EYES. MY MOTHERS MOTHER HAS EXTRAORDINARY ROBIN-EGG BLUE EYES. I DON'T KNOW THE OTHER EYE COLORS OF IMMEDIATE ANCESTORS BUT I THINK SOME HAVE HAZL AND BROWN AND BLUE. WHY ARE MY EVES BRIGHT BLUE?


marie 6 years ago

There is no way two genuinely blue eyed parents could produce a child with brown eyes. It's genetically impossible. Are you sure one of your parent's eyes aren't hazel?


Chris 5 years ago

It's not impossible to have a brown eyed child from a blue eyed parent. There is more than one gene that when mutated gives blue eyes. There is one as described in the Ira case study above and another, say H. h/h will give blue eyes as bl/bl give blues eyes. But the blued eyed person with h/h genotype may well have two Br alleles. The blued individual with bl/bl may well have two H alleles.

Their children would be heterozygous for both genes Br, h/bl, H. Since they have one functional coipy of both genes they will now have blue eyes.


Chris 5 years ago

It's not impossible to have a brown eyed child from a two blue eyed parent.

There is more than one gene that when mutated gives blue eyes. There is one, as described in the Ira case study above, and another, say H. The h/h genotype will give blue eyes as bl/bl give blues eyes. But the blued eyed person with h/h genotype may well have two Br alleles. The blued individual with bl/bl may well have two H alleles.

Their children would be heterozygous for both genes Br, h/bl, H. Since they have one functional copy of both genes they will now have brown eyes.

The bl allele is much more common in the population than the h allele. So normally when two blue eyed parents have children they both have the bl alleles and the H alleles. This explains the misconception that blued eyed parent cannot have brown eyed children because most of the time that is the case.


s.roobashini 4 years ago

This is very helpful for us.

This is very useful for long days.


D.d 4 years ago

This is far to oversimplistic. It has been established for some time that eye colour is determined by more than one gene. PLEASE stop publishing this BS, it is undermining the hard work of science and biology teachers everywhere!!


Glindy 4 years ago

My Mother has blue with a light brown speck in one eye, my father brown. My husbands mother brown,and his father blue.My husband and I both have brown eyes is it possible for us to have a blue eyed child?


anon. 4 years ago

Eye color is determined by more than one gene at more than one locus. It is not a matter of simple Mendelian genetics. Therefore, this model does not accurately explain eye color variation. It is entirely possible for both parents to have blue eyes and produce blue eyed offspring.


Melissa 4 years ago

I have 2 sons with different eye colours than both my ex husband and myself. We have blue eyed and almost black eyed sons. I have hazel and my ex is green. So the theory is not always correct....


webmistris 4 years ago

A black man except possibly an African CAN have a BR bl genes as most American blacks are mixed with bl recessive or dominant genes.


Jane 4 years ago

my eyes arent dark brown, they have some green is it hazel??


baby.doll 4 years ago

Jane yea u have hazel eyes! :)


baby.doll 4 years ago

for everyone who thinks that 2 blue eyed people can't produce a brown eyed baby,i;d suggest you to go t0 this web http://sciencecases.lib.buffalo.edu/cs/files/eye_c...

It helped me..so :)

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