MadSci Network: Genetics

Re: What causes Llamas to be born with one blue eye?

Date: Fri May 28 17:14:53 1999
Posted By: Christopher Carlson, Grad student Genetics
Area of science: Genetics
ID: 926902810.Ge

About a year ago I answered a similar question re: humans with eyes of different colors. At the time I wasn't able to dig up much literature on the subject, and in the interim I still haven't found much. However, some new information has become available which may be tangentially relevant.

First, in humans the condition is known as heterochrom(i)a. It is uncommon, but not exceedingly so. For example, the singer David Bowie and the actor Christopher Walken both have one hazel eye and one blue eye. The medical establishment hasn't really investigated the inheritance of heterochromia, presumably due to its generally cosmetic nature. I did find one reference (Scheie & Albert: Textbook of Ophthalmology p. 295) which claimed that the condition is inherited as an autosomal dominant trait, but have not found support for such a claim anywhere else in the literature.

A related condition where a single iris has segments of two different colors is known as bicolor iris. This will be important later.

Some old information of relevance is the basic metabolism of the pigment melanin. Melanin is the pigment which colors hair, skin and eyes in mammals. It has two flavors: eumelanin which is black/brown, and phaeomelanin which is yellow/red. Both are produced from a single precursor molecule. The genes which catalyze the production of eumelanin and phaeomelanin have been characterized, and mutants in these genes have been shown to produce black, yellow and brown mice of various shades, and recently alleles of the same genes have been shown to account for red hair in humans.

The iris is a muscle which regulates the amount of light admitted to the retina by contracting in bright light or expanding in dim light. In humans and most mammals it is pigmented. Significant variation is observed in this pigmentation, from Blue (no pigment) to green (some yellow pigment) to hazel (lots of yellow pigment) to brown and nearly black (lots of brown pigent). The alleles carried by an individual of genes in the melanin producing pathway determine what color the iris becomes.

Now the fun part: how can a single individual have eyes of two colors? The genetic makeup should be the same in each eye, right? One hypothesis would be that there are regulatory pathways specific to each eye. Genes controlling the left right asymmetry of the internal organs in mammals have recently been described (think of your heart: it is asymmetric), but such genes are rather unlikely to have a role here. Using Occam's razor, nature is unlikely to have two sets of genes for eye development when one will do.

More likely is the possibility that there is an unstable regulatory element in one of the genes coding for part of the melanin synthetic pathway. The Nobel Prize was awarded to Dr. Barbara McClintock for describing such elements in corn, now known as transposons: as a stalk of corn carrying such elements grows from a single seed, whenever a transposon jumps the color of the kernel changes from yellow to blue. If the transposon jumps very early in development, it will result in a large blue patch. If it jumps later in development it may result in a single blue kernel, or even a yellow kernel with blue speckles if it jumps very late in development.

My bet is that there is a transposon near an eye specific regulatory element in the melanin pathway. If it jumps early in development, then the animal will have heterochromia, but if it jumps later it will result in bicolor iris. Which eye is colored should be random. Because the transposon is not guaranteed to jump in every animal, the trait is unlikely to breed true in the purest sense, but should generally be dominant over blue eyes and recessive to brown eyes.

In addition to llamas, some breeds of dogs have a high incidence of heterochromia: I'm personally familiar with malamutes and australian shepherds, and there are probably other examples out there. Testing hypotheses re: the inheritance of heterochromia in dogs or llamas could be a really fun high school biology experiment.

Chris Carlson

Current Queue | Current Queue for Genetics | Genetics archives

Try the links in the MadSci Library for more information on Genetics.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-1999. All rights reserved.