Students of high school inheritance often want to know more about how humans inherit eye color. This presentation goes into depth beyond what high school students typically need to know.

1. Human Eye Color
Inheritance Patterns

2. Eyes
Eye color is controlled by at least three genes (three are known).

3. The Cause of Eye Color
There are two types of melanin protein found in the
iris of eyes; yellow and black. In combination, these
proteins combine to yield eye color ranging from dark
brown to blue.

4. Blue Eyes
A lack of melanin in the iris is pure blue eyes.

5. Green Eyes
Green eyes result from intermediate amounts of
melanin in the iris.

6. Brown Eyes
A lot of melanin in the iris makes brown eyes.

7. Hazel Eyes
Hazel is an umbrella term for mixed eyes. They have
both green and brown areas, in any mixture.

8. No Melanine (Albino)
A complete lack of melanin in the iris is albino, and the
iris appears pink due to the blood beneath the surface.

9. One Gene, Two Alleles
BB Bb
Bb bb
B
b
B b At times, we treated eye
color as simple
dominance.
Of course, if this were
true, about 25% of
humans would have blue
eyes and there would be
no green eyes.

10. One Gene, multiple alleles
BB Bb
Bb bb
B
b
B b
BB Bb
BG Gb
B
G
B b
If eye color were controlled by multiple alleles, we might
find three eye colors, but there would be little variation
in shading, and no hazel eyes.

11. Polygenic, Multiple alleles and
incomplete dominance!
BBbb
BBGb
BBGG
Bbbb
BbGb
BbGG
bbGG
bbGb
bbbb
Brown
Brown
Brown
Brown
Brown
Brown
Green
Green
Blue
Note that these are not
all of the possible
combinations of the
alleles B, b and G.

12. Dominant Brown Eyes
Still, some patterns are clear.
Brown eyes are dominant to
green or blue.

13. Don’t It Make my
Brown Eyes Blue
In 2008, Hans Eiberg released a study that identified a
specific mutation on chromosome 15 that most likely lead
to blue eyes between 6,000 and 10,000 years ago.
The mutation causes a decreased
production of melanin in the iris

14. Don’t It Make my
Brown Eyes Blue
Definition of specific: Every blue-eyed person
he tested had the exact same mutation.
Humans are not the only
organisms that might have this
kind of mutation.
The mutation is in the OCA2
region of the HERC2 gene and
creates most of the differences
in eye color.

15. Green Eyes
Revisited
Green eyes are believed
to be caused by a gene
other than the OCA2
gene.

16. If you see eyes this color, don’t trust them!

17. Heterochromia
Examples of heterochromia – one
blue eye and one brown eye.
David Bowie

18. Heterochromia
Heterochromia is usually acquired, rather than genetic.
An injury to a fetus may
block melanin from
reaching an area of the
iris, or the entire iris.

19. Heterochromia
Officially, heterochromia is the condition of having two
colors in the iris (distinguished from hazel). But it
doesn’t have to be two different eyes; it can occur in
the same eye.

20. And yes, this isn’t restricted
to humans.
And yes, it’s still more
complicated than we
give it credit for.