View Full Version : Are brown eyes allways dominant...
05-07-2003, 08:28 PM
in a brown/blue pairing?
05-07-2003, 08:36 PM
Nope. My husband has brown eyes, and I have blue eyes. Our two sons have blue eyes. Our daughter has brown.
My oldest son from my first marriage has blue eyes. His dad has hazel (more brown than green) eyes.
05-07-2003, 08:42 PM
Thanks, I thought as much, but I'm gathering some evidence so let me re-phrase the question.
My dad and I both have blue/green eyes, my mom and sister both have brown. It was suggested that my parentage my be questionable based upon this. I dont know the first thing about genetics, can someone give a crash course?
05-07-2003, 08:51 PM
Silky: You are confusing phynotype (appearance) with genotype (genetic makeup). It's quite possible that your brown eyed husdband has blue eyed alleles (thanks to Colibri for reminding me about alleles today in my "99.9% geneticall the same" thread). Eye color is thought to be determined by several genes, so it gets pretty complicated.
We're all used to the simple one gene concept we learned in jr high: Tall beans crossed with short beans:
BBxbb=Bb. (B=tall; b=short).
Then BbxBb=BB, Bb, or bb.
Bb is still tall, but two tall beanstalks can prduce short (bb)
05-07-2003, 08:53 PM
Oops. Didn't mean to imply you were incorrect, Silky. Actually our posts say pretty much the same thing. Sorry-- poor editing.
05-07-2003, 09:01 PM
Isn't that what I said?? ;)
I am not sure of my first husband's genetic makeup (hair or eye color) but according to everyone in both sides of my present husband's family, up to his now-deceased great-grandmother, there are no blue eyes in that family. But of course, there could be beyond her.
Actually, I really did know that about the recessive genes thing, but thought I would just cut to the chase and say, Nope!
I am certainly no geneticist! There's no way I could have answered in that equation, John Mace. I appreciate your much more thought out response.
05-07-2003, 09:03 PM
And poor non-previewing on my part...!
I can't believe it. I actually may have said something right (sorta-kinda) for a change on the boards.
Mark this down, kids! You'll never see it again!
05-07-2003, 09:03 PM
The crash course version.
Blue eye colour isnít really encoded genetically. Itís just a default colour that you get when there is no pigment found in the iris. In other words the iris is naturally blue by default and has to be coloured to be anything else.
You get two copies of the eye colour gene, one from each parent.
If you have two Ďdamagedí copies of the gene then you have no capacity to produce pigment in the eye. The iris retains its base colour: blue.
If even one of your copies is working then there will be pigment produced in the iris. Since this brown pigment is darker than blue it will hide the blue colouration. Your eyes will be brown.
If you have two working copies then eye will still be brown.
In this case your mother has brown eyes even though she has only one working copy of the brown gene. But thatís all thatís needed to colour over the Ďnaturalí blue.
Your father has two defective copies of the eye pigment gene and so has blue eyes.
You inherited one of your fatherís defective genes and also got the defective copy of your mothers two genes. So you have two defective genes and canít produce pigment in your eyes.
Your sister inherited one defective gene from your father, but she got the working copy from your mother. As a result she produces pigment in here yes, and the brown pigment has swamped the Ďnaturalí blue.
No problems there genetically. If your parents had any more children then statistically half of them should be brown eyed and half blue.
Note that this is the extremely simplified version. Human eye colour isnít simple dominant recessive. Nor is it dependant on just one gene pair. Itís close, but it doesnít quite work like that. This allows people to have an infinite variety of eye colours including hazel, violet and green as well as blue and black. It also allows peopleís eye colour to change, usually getting darker, as they age. It also allows two people with blue eyes to occasionally have brown eyed children, which would be impossible with a simple dominant/recessive mechanism.
But no matter what the mechanism, so long as one of the parents has brown
Eyes the children can theoretically have any eye colour at all.
05-07-2003, 09:10 PM
There are no blue eyes in my family, nor in my wife's, yet our daughter had blue eyes.
Eye color is more complex than just dominant/recessive. This site (http://www.seps.org/cvoracle/faq/eyecolor.html) discusses the issue. There are three genes controlling eye color, and the model only covers blue, green, and brown eyes. Gray or hazel eyes don't fit.
This site (http://www.athro.com/evo/inherit.html) has a calculator that shows how two brown-eyed parents can produce blue-eyed children.
05-07-2003, 09:15 PM
Originally posted by Blake
Blue eye colour isnít really encoded genetically. Itís just a default colour that you get when there is no pigment found in the iris. In other words the iris is naturally blue by default and has to be coloured to be anything else. Then why is it that albinos, who have no pigment in the iris, do not have blue eyes? Albinos always have pink irises. This would lead me to believe that blue eyes are, indeed, caused by pigmentation.
Is my reasoning flawed?
05-07-2003, 09:33 PM
Youíre quite right about albinos. The pink is caused by a total lack of pigment allowing the blood vessels to be seen. Like I said, the above is the simplified crash course version. However the blue colouration, while itís produced by proteins encoded via genes etc, isnít really genetically encoded in any way thatís meaningful to a crash course in genetics. Barring major defects like albinism that prevent any pigment being produced anywhere on the boy, the blue colour is a default. Hence I can say itís not Ďreally; genetically encoded.
People who suffer genetic defects causing them to be born without eyes also donít have blue as the default eye colour, but like albinism this is so far removed form the normal human condition that itís beyond the scope of a 200 word crash course.
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