I.e., GENES EXPOSED FOR WHAT REALLY FITS:
" EYE DREAM
OF GE-NE..."
(WITH MY LIGHT
BROWN HAIR)
How Blue Eyed Parents Can Have Brown
Eyed Children
yes, that's what I said !!
(Our grandson Jim...the product
of blue-eyed parents)
It's always been known that there are
two different ways to get blue eyes...
but what about brown?
Three of our four
children have eyes that would generally be described as "blue" and
one has definite brown. The funny thing is that neither of my husband's parents
have blue eyes, yet his father's mother and his mother's father most positively
DID. As regards my own parents, my father was brown-eyed and my mother's
eyes were aqua-blue. Mine turned out turquoise, my middle sister's are brown
and my youngest sister's are pale aqua. Now, I would have thought that my
daughter's chance of having a truly brunette child were slim to none, when
assessing her husband's coloring. That said, 'his' mother is brunette as is my
husband. Still, MY offspring's eye color results are probably not as unlikely
as my grandson's outcome.
Punnet squares and
Hardy-Weinberg, notwithstanding ... the genetics of eye color is much more
intricate than what is usually taught in high school or even a college genetics
course. Blue eyed parents CAN have a brown eyed child, though this
phenomenon is certainly not common! Brown eyes, as most will recognize, are
the result of genetic dominance. However, when 2 genes come into play for eye
color, things get shaken up a bit ... and here is where I will hand the
metaphorical baton off to a more professorial discussion of the subject:
The take
home point is that if someone makes a lot of pigment in the front part
of their eye, they'll have brown eyes. And if they make none there, they
have blue. This insinuates that blue eyes are the result of "broken
genes". Ergo, there are two ways to produce blue eyes via the above
schematic.
The pigment making process involves both OCA2 and HERC2. A
'working HERC2' is needed to turn on OCA2, and OCA2 helps to actually get the
pigment made: in other words, they truly need each other to make pigment.
So someone with only broken HERC2 genes will have blue eyes no
matter what OCA2 says or does. This is because the working OCA2 can't be
turned on so no pigment gets made.
The following is true as well. Someone with broken OCA2
genes will have blue eyes no matter how 'normal' the HERC2 gene is.
Turning on a broken pigment making gene still gives you no pigment. You
need BOTH a working HERC2 AND a working OCA2 to have brown eyes.
But here is the caveat: Because the two genes depend on each
other, IT IS possible for someone to actually be a carrier of a dominant trait [like
brown eyes]. And if two blue eyed parents are carriers, then they CAN
have a brown eyed child!!!
Carrying a Dominant Trait
People
have two copies of most of their genes. They get one copy from their mom
and one copy from their dad.
These
genes can come in different versions (known as alleles). For the sake of simplicity
and clarity, let's say that OCA2 comes in brown (O) and blue (o) versions, even
though reality is actually a bit more complicated. Moreover, let's also say
that HERC2 comes in two different versions, brown (H) and blue (h). Now, since
people have two copies of each gene, there are nine different possible genetic
combinations with the array of alleles.
Furthermore, these different combinations give
the following eye colors:
Looking at the table, note
that any time there are either two lower case h’s or two lower case o’s, a
person has blue eyes ... and [again] this is because HERC2 and OCA2 need each
other to have an effect; i.e., the blue versions of each gene are essentially broken.
But now let us focus on how two blue eyed
parents might have a brown eyed child. Imagine the following blue eyed
parents:
Why OCA2 and HERC2 TRULY
Need Each Other
FACT: Eye color depends on how much pigment is in the eye.
A lot of pigment gives brown, some gives green and little or no gives
blue. OCA2 is one of the key genes in determining how much pigment gets
made. If both OCA2 copies are broken, someone would have blue eyes.
Yet, most of the HERC2 gene has very little to do with eye color, per se.
That said, there is one small [critical] section in the middle of this gene that
controls whether OCA2 is turned on or not.
If this part of HERC2 ends up broken in both copies, then OCA2
can’t get turned on. And if OCA2 is off, no pigment gets made. As a
result thereof, it is "like" the OCA2 gene is broken.
CASE IN POINT: Think about OCA2 like a light bulb and HERC2 as a
switch. If the light bulb is burned out, it doesn’t matter if the switch
is turned on, does it?. Just like it doesn’t matter if HERC2 works in
someone with broken OCA2. Flipping the switch to a burned out bulb won’t
give you any light either!
Same thing with a working OCA2 and a broken HERC2. A
working light bulb gives no light when the switch is off. Similarly, a
working OCA2 makes no pigment when the HERC2 gene is broken.
This is what happens when blue eyed parents carry a brown eye
gene. If they have blue eyes because of a broken HERC2, then they might
still have an OCA2 that works! And if they have a broken OCA2, they might
have a working HERC2!
Here is how to think of it in terms of these two parents passing
on their genes to the next generation:
When
these two have kids, one might pass an on switch and the other a working light
bulb. Now there is light even though neither parent could make light
before. Or in genetic terms, one might pass a working HERC2 and the other
a working OCA2. Now there is pigment where there wasn't any
before. The end result is brown eyes.
BUT
WAIT...there's more: OCA2 and HERC2 are Linked!!
An
interesting twist to this puzzle is the fact that HERC2 and OCA2 are very close
together on chromosome 15. What this means is that versions tend to
travel together in the manner of linked genes. And this affects the
combinations of kids that any two parents can actually have.
To reiterate: Think
about OCA2 like a light bulb and HERC2 as a switch. If the light bulb is
burned out, it doesn’t matter if the switch is turned on. Just like it
doesn’t matter if HERC2 works in someone with broken OCA2. Flipping the
switch to a burned out bulb won’t give you any light! Same thing with a working
OCA2 and a broken HERC2. A working light bulb gives no light when the
switch is off. And a working OCA2 makes no pigment when the HERC2 gene is
broken. This is what happens when blue eyed parents carry a brown eye
gene. If they have blue eyes because of a broken HERC2, then they might
still have an OCA2 that works. And if they have a broken OCA2, they still
might have a working HERC2.
****************************************************
Pretty array of
capacitors, don't you think? There are some that will still take physics over
genetics any day!
(...
and then there are those who truly enjoy their puns.
In one memorable Loony Tunes episode,
Bugs
Bunny clearly emotes,
"I dream of Jeannie, she's a light brown
hare").