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