Coloring the Eyeby Louise Callanta

“The eyes are the windows to the soul.”

Mendelian Genetics

• Genetics are normally shown in the most simple terms. Look at Mendel and his pea plants. By looking at the parental lineage, you can predict what the child plants will look like.

• Humans, in like terms can be predicted to look similar to their parents. But the DNA coding, has complications lurking unseen waiting to be discovered.

Using the Punnett Square

I have blue eyes, and my husband has brown eyes, so by using on simple Mendelian genetics, I can predict the color of eyes for our potential children.

B B

b Bb Bb

b Bb Bb

B b

b Bb bb

b Bb bb

or

The Punnett squares leave me to expect to have brown eyed or blue eyed children.

The Prediction vs. The ResultsWhen my oldest child was born I expected a possible of two outcomes for her eye color; brown or blue. I didn’t expect green eyes.

Where did they come from?

Well her parents of course, but how?

Taking a new look

Green is a dominant color to blue eyes, and recessive to brown eyes. My initial Punnett Square predictions were missing information, I didn’t realize a green genome was lurking.

B G

b Bb Gb

b Bb Gb

I thought I had solved the mystery of the green eyes, and expected for future children to have brown eyed or green eyed children. Then my next child came along, and I was blindsided with blue eyes.

How did the blue recessive gene trump the dominant brown and

green genes?

My husband has suggested on a few occasions that maybe the blue eyes came from the mailman. Which of course I am kind enough to inform him that our mail carrier is female and an unlikely sperm donor.

But the question has still remained for many years, how do eye genetics work? How is it possible to have multiple potential eye colors? Why didn’t what I was taught in high school apply to the real world?

The easy answer is simplification, genetics are not simple, but have been taught in a simplified manner to help students understand the basics. And in my case, causing a bit of confusion down the road.

Digging deeper

(Picture from Athro.com)

Genes for eye color have been found on Chromosome 15 and on Chromosome 19.

Genes for human eye color are: EYCL1 (gey) for green/blue eyes which is found on chromosome 19, EYCL2 (bey1) for brown eyes which is found on chromosome 15, and EYCL3 (bey2) for brown/blue eyes and also located on chromosome 15.

Other eye colors have not been explained yet. It is hypothesized that alleles for eye color may be on as many as 16 chromosomes.

The picture below shows the EYCL3 gene (bey2) on chromosome 15 with respective color alleles. Brown eyes are the Phenotype in this example.

The less-simplified Punnett squareNow that I have learned that genes for eye color happen on multiple chromosomes and have been identified on chromosomes 15 and 19, I can create a Punnett square that more accurately reflects my potential offspring. It also reflects why the simpler form is normally used.

Parent 1: brown eyesGenotype: bey2: brown-blue, gey: green-blue

Parent 2: blue eyes

Genotype: bey2: blue-blue

gey: blue-blue

bey2:gey:

Brownblue

BrownGreen

blueGreen

blueblue

blueblue

Brown-blueblue-blue

Brown-blueGreen-blue

blue-blueGreen-blue

blue-blueblue-blue

blueblue

Brown-blueblue-blue

Brown-blueGreen-blue

blue-blueGreen-blue

blue-blueblue-blue

blueblue

Brown-blueblue-blue

Brown-blueGreen-blue

blue-blueGreen-blue

blue-blueblue-blue

blueblue

Brown-blueblue-blue

Brown-blueGreen-blue

blue-blueGreen-blue

blue-blueblue-blue

With the creation of the complicated Punnett square, I can more accurately see the potential phenotypes for potential children. I have a 50% chance of having children with brown eyes, a 25% chance of having children with green eyes, and a 25% chance of having children with blue eyes.

The 50% chance of brown eyes, can be broken down even smaller because half carries the green and blues genomes as recessive traits, and the other half only carries the blue genome as recessive traits.

The examples I have shown reflects the assumed genomes for eye color has worked for my own personal situation. Other people could have similar results to mine, or totally different based upon their own genetics and their partner’s genetics.

Hidden DNA Surprises Is it possible for a child with two blue eyed parents to have brown or green eyes?

It may go against everything you have been taught about dominant and recessive genes, but there are people who have brown and green eyes with both their parents having blue eyes.

How does a person with a recessive phenotype carry a dominant genotype?

Rarely, a dominant eye color will show up from recessive parents.

The explanation is very simply that one of the parents does indeed carry the dominant genome. The reason why the recessive genome is the phenotype, instead of the dominant genome is because of a mutation.

During reproduction when half of one parents genes are combined with the other parents genes the mutation is fixed and so a hidden dominant phenotype is revealed.

Other MutationsOffspring can also have eye colors that are not genetically passed on. It is not fully understood why these mutations take place. Drug use has been documented as a reason to change DNA. There are more unknown reasons than known ones.

Another mutation is Albinism which produces red eyes. This has been associated with lesions on a gene called TYR.

Shades of eye colorsEye colors range from red to black. What has been found to cause the great variations in colors and shades of the eye is the melanin in the eye. Melanin pigments the eye, just like it pigments the skin. People whose bodies create little or no melanin (albinism) have red eyes. The less melanin in the eyes the lighter the eye color. The more melanin the darker the eye color. The is also a correlation that most people with darker eye colors tend to have darker hair and skin, whereas most people with lighter eye colors have lighter hair and skin. Although, like everything else there are exceptions to the general trends.

Eye color itself comes from a combination of two black and yellow pigments in the iris of the eye. If there is no pigment (melanin) in the front part of the iris, the result is blue eyes. An increasing proportion of the yellow melanin, in combination with the black melanin, results in shades of colors between brown and blue, including green and hazel.

No Easy AnswersHow grey, hazel and black eyes are determined genetically has not been identified yet. There is still a lot of mystery in relation to how eye color genomes and phenotypes work. Eye color is believed to be spread among as many at sixteen chromosomes.

Knowledge about DNA is constantly growing. The double-helix of DNA was identified in the 1950’s, it is amazing how much knowledge has come forth in 50 years since this discovery. It is exciting is that there is a lot more to be discovered about DNA and genetics, so more questions can be conclusively answered.

Finishing upI decided to write about eyes based upon my own questions about genetics. When I was in high school I recall asking my science teachers questions that they did not have the answers for. In looking into these answers, I now understand why they didn’t have the answers; there weren’t any. Even now, with the increased knowledge about DNA, finding information on this subject has been quite difficult.

I also feel this project flowed with the knowledge I gained while studying it. I did find some fun resources I was not able to fully incorporate, but here is the link to one of my favorites: http://www.athro.com/evo/inherit.html. You can put in your own eye colors and make predictions based upon different genetic factors. Overall I hope you have gotten out of my presentation some understanding about human genetics and how complicated they really are. I chose a relatively simple subject, to show how complicated human genetics are is overall.

Works Cited• Frudakis, Tony, Zach Gaskin, et al. "Sequences Associated with

Human Iris Pigmentation." Ed. P. J. Oefner. Genetics 165 (2003): 2071-2083.

• Morris, Paul J. "How are Human Eye Colors Inherited?" Athro Limited. 9 June 2000. 1 May 2006 <http://www.athro.com>.

• Starr, Barry. "Understanding Genetics; Ask a Geneticist." The Tech Museum of Innovation. 23 June 2005. Stanford University. 4 May 2006 <http://www.thetech.org>.

• "The Genetics of Human Eye Color." Science Education Partnerships. 16 Mar. 2006. Oregon State Univeristy. 5 May 2006 <http://www.seps.org/>.