1 Brown/1 Blue eye

anthonyp at scripps.edu anthonyp at scripps.edu
Mon Nov 27 14:34:32 EST 1995


In article <bohmfalk-2611951308220001 at pm1-8.tcgcs.com>, 
<bohmfalk at tcgcs.com> writes:

> I have a student who has one beautiful brown eye, and one beautiful blue
> eye.  Do any of you know the genetic/molecular mechanism behind this
> phenomenon?  Any idea how common it is?  I would appreciate any
> information, or references.
> 
> Thanks all.
> 
> John Bohmfalk

Aside from the epigenetic phenomena someone else offered (drug side 
effect, or alterred expression of pigment deposition), I can think of two 
genetic causes.  
In general terms, the person is a "mosaic," that is, expressing different 
genotypes in different cells.  This can arise several ways.  The person 
would have to have been heterozygous at the locus involved (Br/br) and 
therefore brown-eyed phenotypically.  Somehwere after the lineages for the 
two eyes had diverged, a somatic cell event occured to alter the genotype. 
 
1) The Brown allele was lost (due to a mutation or non-disjunction). This 
results in the person being hemizygous (br/-) for that locus in some 
patches of cells, including the blue eye's iris. 

2) the Br allele was converted in a somatic cell recombination event to 
the br (blue) allele, resulting in patches that are homozygous br/br.

3) related to 1 above.  In families in which this is a common trait (or 
breads of dogs) some other trick must come into play.  The animal could be 
br/br at the normal locus, and have a third copy of the gene (arising 
from a translocation or a duplication) in the Br allele at another locus. 
 This third allele is for some reason either unstable and therefore lost 
in some cells, or is inactivated in some cells.  
The latter is common if the third locus in on the X and the animal is a 
female.  All female mammals are mosaics for genes on the X, because one X 
chromosome is inactivated (or nearly so) in each cell in the body.  In 
some cells, the paternal X is expressed and in some the maternal.  This 
only happens in females or XXY males.

Another possibility happens in other species of animals, but I have only 
heard anecdotal mention of it in humans.  The person could be a chimera 
resulting from the fusion of two fertilized gametes--sort of the opposite 
of identical twins--fraternal twins that fused.  I don't know if this 
really can happen in humans.  If so, it would open all sorts of odd 
possibilites

The mosaic explanation seems more likely, but I don't know which mechanism 
is in play.

-tony



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