[email protected] (Wirt Atmar) wrote in message news:<
[email protected]>...
> Craig asks:
>
> >I tried to find a genetics chatroom but I thought maybe someone could answer me. My son who is 2
> >months old has one brown eye and one green-grey eye. It turns out my wife's aunt has the same
> >thing. Is this common? This gene would probably be recessive, so does this mean I'm also a
> >carrier? I have brown eyes and my wife has green eyes. My son's twin sister has two lighter
> >brown eyes.
> >
> >I think David Bowie has this and I heard Alexander the Great also had this. Any other famous
> >people?
>
> This is a recurring question here on this newsgroup. Enclosed below is how I answered the question
> approximately four years ago:
>
> ======================================
>
> From: Wirt Atmar (
[email protected]) Subject: Re: Human eye -- weird pigmentation Newsgroups:
> sci.bio.evolution Date: 2000/01/05
>
> Louann Miller asks:
>
> >Just a quick question, probably not controversial in any way but an oddity I've been wondering
> >about over time.
> >
> >I had a college classmate whose left eye was two different colors. The iris was divided into two
> >distinct half-circle shapes, blue on one side and light brown on the other. The dividing line
> >(vertical) was sort of greenish. Her right eye was the same shade of blue as the blue half of her
> >left eye.
> >
> >She said it had always been like that, it didn't hurt, and she didn't have any vision problems. I
> >didn't think to ask if anyone else in the family had it. It was weird enough to be memorable.
> >I've never met or heard of anyone else with the same condition (except a fictional character in
> >George MacDonald Frazier's "Flashman" novels, which hardly counts.)
> >
> >So: what is the name of this condition? Is it genetic or something in the prenatal environment?
> >Is is related to the more common condition where a person (or a cat or dog) can have each eye a
> >single different color? Etc. I'm interested in any information anyone has.
>
> The general morphological description is called "heterochromia irides", which means nothing more
> in plain English than "differently colored irises."
>
> In that, the condition is not normal, and even though the bearer may be completely non-impeded by
> its presence, it is a genetic "defect." However, it is often more serious than that. Heterochromia
> irides, especially when it refers to eyes of two different colors -- but sometimes in eyes where
> one iris is either splotchy with both blue and green/brown colors or evenly divided into two
> differently colored hemispheres -- is a primary diagnostic character for Waardenburg Syndrome
> (see:
http://www.icondata.com/health/pedbase/files/WAARDENB.HTM ), a developmental abnormality
> that is common to many large mammals, exactly as you suggest.
>
> Humans, cats and dogs that exhibit eye colors that are quite markedly differently colored, where
> one eye is blue and the other green or brown, are often mildly to profoundly deaf. The abnormality
> appears to be due to a point defect in the pax-3 gene, a homeo box gene that is widely expressed
> during early embryogenesis, thus explaining the wide range of effects that the gene has.
>
> Waardenburg Syndrome is inherited dominantly, however it is suspected that only perhaps 20% of
> those bearing some form of the defect are actually aware of it, its effects being so mild in
> most people.
>
> More information can be found at:
>
>
http://www.boystown.org/deafgene.reg/waardsx.htm
>
> Wirt Atmar
>
> ======================================
>
> Wirt Atmar
There is no reason to panic. Waardenburg is not the only reason for these eye color changes. I
worked with MITF a couple of years ago and if there were a reason to panic you would know already.
There would be a family history of this disease unless you had one of the mild alleles. It is
inherited as a dominant trait. Not only that, but only 2% of the cases of genetic deafness are
Waardenburg. This is a significant portion of deaf cases, but it isn't a significant portion of the
population.
>From what I recall not all Waardenburg patients exhibit this
phenotype. Certain MITF mutations have a 5 to 8 times higher rate of showing occular color changes.
This tells me that most of the Waardenburg patients may not have the eye color change. It is
associated with the disease. The reason for the occular color changes is just what I posted. It may
be a dominant for clinical symptoms, but it isn't a dominant for pigmentation changes. It probably
shows a higher than normal rate of these color changes because PAX and MITF are very important genes
and messing them up affects a lot of genes. There are very many more somatic mutations that will
cause a color shift for MITF than the normal blue eyed alleles found among humans. There are orders
of magnitude more normal (nonlethal) blue eyed heterozygotes than MITF or PAX blue eyed
heterozygotes. MITF and PAX mutations are not often found in the homozygous condition and when they
are they exhibit extreme phenotypes as well as blue eyes.
What seems to be happening is that one good copy of MITF is all you need to produce melanin, but in
some of your cells mutants or recombinants happen and you get two bad copies or you lose the good
copy. When this happens in the eye you get blue instead of brown. In the case of MITF, messing up
some other gene like tyrosinase is known to cause the color shift. So color shifting will probably
occur at a higher frequency than for other blue eyed alleles.
I have never seen the number of normal people with occular color shifting compared to Waardenburg
patients. I am sure that the frequency will be much higher in Waardenburg patients, but it will not
be zero among normals because of the high frequency of normal (nonlethal) blue eyed alleles in the
population. I expect the overall frequency of color shift due to normal blue eyed heterozygotes to
be higher in the population than those due to Waardenburg because Waardenburg is such a low
frequency in the population, but I've never seen the numbers.
If you are worried about this. Check out an expert on Waardenburg.
.