Is blue eyes and red hair all in the genes or is there something more going on behind
this rare combination?
In 2008 South East Queensland Australia, a young redheaded girl and her twin enrolled
in a scientific study.
When the researchers tested their DNA, they made a brand new discovery in human genetics.
As reported in the Human Mutation journal, the DNA of one of the little girls told scientists
the default hair color for all people is red.
I know, surprising huh?
It seems strange that the default color should be so rare.
In fact, depending on the population, the British Medical Journal reports that only
1% to 2% of people have red hair.
So what's the reason behind it?
Genetics.
Genetics is the main culprit responsible for both hair and eye color.
In fact, your genetics has roughly 61% to 100% to do with hair color and 98% with eye
color according to the Biological Psychology journal.
With both red hair and blue eye traits being rare worldwide, you might think it would be
the rarest combination.
But the genetics of eye color is more complicated than recessive or dominant traits.
The complex nature of genes and protein interactions is keeping the secrets behind these traits
all knotted up.
We owe the color of our hair and skin to cells called melanocytes and the pigment they produce.
These cells, as the Human Mutation journal explains it, all make the protein responsible
for red hair: melanocortin-1 receptor, or MC1R.
Different versions of the protein perform their responsibilities at various levels.
Versions of the MC1R genes responsible for red hair are common in the northern hemisphere
and Northern European populations as stated by Annual Review of Genetics.
In caucasians, some versions of MC1R are associated with red hair, fair skin, skin cancer risk,
and freckles.
If you received a red hair version of the MC1R from both parents, you likely have red
hair.
Notably, the red-haired Australian twin didn't have any of the known variations of MC1R.
In fact, the MC1R protein was gone.
She was the first example found of someone with no working MC1R protein.
Given that MC1R is the primary gene associated with the production of the red pigment, the
presence of red hair in the twin — even in the gene's absence — proves that the
default hair color for humans is red.
According to researchers in Pigment Cell & Melanoma Research, the iris is:
"Probably the most complex tissue structure that is readily available on the outside of
the human body."
This is largely because Its tissue, quote, "forms complex patterns that contain many
distinct features," and for this reason, banks, governments, and other security industries
have begun to use eyes for automatic personal identification according to Passport Info.
The iris has multiple layers, and pigments at these different layers are part of what
gives the iris its color.
For example, brown eyes have lots of melanocytes and therefore higher amounts of melanin than
blue eyes.
The thickness of the iris also affects color.
It determines if you're sporting blue-gray, green, or hazel eyes.
Is the combination of red hair and blue eyes in one person really the rarest?
Some say that it's as simple as them both being recessive genes and having a low prevalence
in the world population.
However, the relationship between hair color and eye color is complex.
The science behind hair and eye color is as entangled as curly hair after a windstorm.
While articles such as one in the University of Melbourne's Pursuit state that the lowest
odds for any hair-eye color combination is red hair and blue eyes at 0.17%, a scientific
article verifying this number is difficult to find.
In a caucasian Netherlands population, where more redheads can be found for their study,
researchers in Twin Research and Human Genetics found that 1.6% had the rare red hair and
blue-gray eyes combo.
However, the green-hazel eyes and red hair combination was lower at 0.4%, and red hair
and brown eyes was the lowest prevalence at 0.3% of the study group.
This shakes up the assumption that if both traits are recessive and at low prevalence,
they're going to be the rarest in combination.
However, further research is necessary because this study was limited by the lack of redheads
— even in their 3,500-plus subject pool.
Given that red hair is the default hair color, it will be interesting to see what future
research reveals about the rarity of red hair and blue eyes — and what other combinations
are even rarer.