Genetics question: Dog's Eyes vs People's Eyes

[danderso8]

Why is it fairly common (quick guess 1/500 or so) to see a dog with one blue eye and one brown eye, but i hardly ever see a person like that? What's different about the genetics there?

thx,
dan

[danderso8]

Aww, c'mon. It's my hundredth post and nobody's even gonna reply? /images/graemlins/frown.gif

[Piz0wn0reD!!!!!!]

a friend of a friend of mine has 1 blue eye and one reddish eye. its really weird.

[Piz0wn0reD!!!!!!]

oh yeah, theres also a poster here who has 2 different eye colors:

http://www.icalledshotgun.com/photodirectory/uw_madtown.jpg

[BluffTHIS!]

Probably because your ancestry isn't heavily mongrelized.

[maurile]

Behold Kate Bosworth:
http://img225.imageshack.us/img225/1659/adv63393lg.jpg

I'm almost positive Jane Seymore looks the same way. One green, one brown.

[purnell]

I think this phenomenon is more common in humans than you are assuming. I have personally known three people with this "condition".

Conjecture: It may not be "mongrelization" but inbreeding that produces this result (I'm from Kentucky lol).

[benkahuna]

Once you understand a bit about gene expression, it becomes fairly clear. I'll hit you with how it happens, but I don't know why there would be a big difference between humans and dogs on this particular matter.


Brief genetics explanation:

Every (nucleated) cell in your body has an entire copy of your genetic code (your genome). In your genome are the instructions for making a number of proteins and which proteins get encoded is partly dependent on the right starting conditions (transcription factors, cell cycle, cues from regional body features, etc.). Depending on conditions, different RNA strands are made (DNA -> RNA = transcription) that correspond roughly to what proteins will be made. They may be copied in a simple manner or different parts of the sequence can be skipped (alternative splicing). These RNA strands may be further manipulated (sometimes part is chopped off) before a ribosome attaches to them and starts making an amino acid chain (a process called translation). After that you have the the amino acid chain that may be further processed and modified (could be pH dependent, need some metal attached to it, need the geometry fixed to work as a transmembrane protein, etc.).

Eye color is controlled by a few genes, not just one. If you have more than one allele for eye pigment (more than one form of an eye pigment gene--two alleles per gene locus except on the Y chromosome), you have the ability to make different pigments. Sometimes, one pigment is more likely to be encoded than another. Usually, stable, functioning proteins result from dominant genes and recessive genes produce less stable, sometimes defective proteins that may ultimately provide a different result (phenotype) or no working proteins at all. If you have a dominant/recessive allelic combo at one gene locus, you may often just make half as much of the dominant gene's protein and none of the recessive. Or, you may get some other mixture with more of the dominant gene's protein but some of the recessive gene's protein. With two dominant alleles at one gene locus, you'll make equal (or close to it) amounts of each protein.

Just to make things weirded and more complicated, some genes (most genes) do not encode proteins, but instead regulate the likelihood particular genes will be active.


Answer to your question (the how, but not the why):

So, for eye color, what must happen is for the conditions to change in expressing the genes for one eye versus the other as obviously genes are expressed at different rates. The different eye colors could result from mutation (a number of environmental factors can cause mutation including UV radiation, viruses, etc.) as damaging the right DNA sequence could damage one or more eye pigment pigments. Very rarely are mutations beneficial. The eye color pigment could be produced at slightly different times and the physical conditions for expression could have changed (pH, ionic concentrations, presence of transcription factors, action of regulatory genes, etc.).


vocab:

genome: your entire genetic code, all (hopefully) 46 intact chromosomes in human

allele: form of a gene. A, B, and O are the 3 common blood type alleles, AO and AA lead to type A; BO and BB to B, AB to AB, and OO to O. Two alleles per gene

expression: whether a gene is on. A gene that is working is said to be expressed and can be a gene product or an active regulatory gene. Gene expression determines what genes are active. The reason the same DNA can make every cell type in the body is that gene expression is different in different cell types.

transcription: turning DNA into a (much smaller) fragment of RNA in preliminary protein manufacture

translation: an RNA fragment is used as a template to make an amino acid sequence, the start of a protein

dominant gene: an allele that is consistently expressed.

recessive gene: an allele that results in a damaged or less efficiently made gene product, not made in quantity unless you have 2 alleles

gene locus: a gene's location, implies there is only one type of gene being made from any one DNA location. Each gene has a spot on its chromosome.

mutation: a spontaneous change in the DNA sequence. In this context it means a mutation to a single gene only

transcription factor: a chemical that changes the rate of transcription. Corticosteroids act as transcription factors, binding DNA and changing the activity level of certain genes.

[sexdrugsmoney]

Facinating.