Transcript Eye Coloring Genetics

Coloring the Eye
by 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
or
B
b
b
Bb
bb
b
Bb
bb
The Punnett squares leave me to expect to have
brown eyed or blue eyed children.
The Prediction vs. The Results
When 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
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.
(Picture from Athro.com)
The less-simplified Punnett square
Now 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 eyes
Genotype: bey2: brown-blue, gey: green-blue
Parent 2:
blue eyes
Genotype:
bey2:
blue-blue
gey: blueblue
bey2:
gey:
Brown
blue
Brown
Green
blue
Green
blue
blue
blue
blue
Brown-blue
blue-blue
Brown-blue blue-blue
Green-blue Green-blue
blue-blue
blue-blue
blue
blue
Brown-blue
blue-blue
Brown-blue blue-blue
Green-blue Green-blue
blue-blue
blue-blue
blue
blue
Brown-blue
blue-blue
Brown-blue blue-blue
Green-blue Green-blue
blue-blue
blue-blue
blue
blue
Brown-blue
blue-blue
Brown-blue blue-blue
Green-blue Green-blue
blue-blue
blue-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 Mutations
Offspring 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 colors
Eye 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 Answers
How 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 up
I 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/>.