biology question #750



Karen Lynn, a 58 year old female from toronto asks on April 12, 2002,

Q:

I have blue eyes and am the mother of a son who has blue eyes as well. His father has brown eyes. My son's father is of East Indian extraction. None of his ancestors appear to have had blue eyes. If brown is the genetically dominant colour and blue the recessive, how did my son come to have blue eyes?

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the answer

Barry Shell answered on December 21, 2006, A:

New information about the genetics of eye colour is here: BBC News: Genetics of eye colour unlocked

There is no specific gene for eye colour. It works differently and is based on single nucleotide polymorphisms, that is the changing of just one part of the DNA chain that codes for a gene that creates the pigment in the iris of the eye.

Brad Davis, geneticist at the University of British Columbia answered on July 23, 2002, A:

Dominant and Recessive in Genetics means something quite different than in their every day usage. A bit of terminology for you first - just to clear things up.

An allele is one copy of a gene responsible for a particular function. Humans are diploid, meaning we have two copies of every gene, one copy from our mom and one copy from our dad. We receive one allele from each parent, for every one of our genes. Every individual has two copies of the gene responsible for eye colour - one from mom, and one from dad. In order for an individual to have blue eyes, because the allele responsible for blue eyes is recessive to the allele responsible for brown eyes, both copies of the gene they have for eye colour must be copies for blue eyes. If an individual has one allele responsible for brown eyes, and one allele responsible for blue eyes, then that individual will have brown eyes. If an individual has two alleles responsible for brown eyes, that individual will still have brown eyes. The only time an individual will have blue eyes is if they have two copies of the gene for blue eyes.

Let me explain it with a small diagram. We're going to let big B represent a copy of an allele that gives you brown eyes, and little b represent a copy of an allele that gives you blue eyes. An individual that has the alleles "BB" would have brown eyes, as would an individual that has the alleles "Bb". An individual that has the alleles "bb" would have blue eyes. Thus since you have blue eyes, you must have both little b's, so you are type "bb". Your husband though, has brown eyes, and he can either have brown eyes because he has "BB" (two copies of the gene responsible for brown eyes) or "Bb" (one copy of the gene responsible for brown eyes, one copy of the gene responsible for blue eyes). Thus when your son recieved his genes from you and your spouse, he necessarily recieved a "b" gene from you, and also recieved a "b" gene from his father. So the reason you and your son have blue eyes, even though his father has brown eyes, is precisely because the gene responsible for brown eyes is dominant to the gene responsible for blue eyes.

If you have one copy of the gene for brown eyes it will mask out the the gene for blue eyes. This is because the gene for brown eyes causes pigment to grow in the iris of the eye which causes them to look brown. As long as you have one copy of the gene for brown eyes, the pigment will be present. If you have only the gene for blue eyes, then the pigment will not develop - and hence you will have blue eyes.

[Editor: It turns out that eye color in humans is more complicated. At least two genes control eye color and they work in complicated ways. There may be more genes involved.]

Eye Color resources on the web:

http://www.wonderquest.com/eye-color.htm

http://www.athro.com/evo/gen/inherit1.html

Shamine Chainani answered on November 14, 2003, A:

Question: I was born with a blue eye and a brown eye and i was never given a proper explaination as to its cause. Can you help me?

Answer: According to Joseph S. Elman, ophthalmology professor at Yale University, several things can cause this abnormality (called heterochromia irides): faulty developmental pigment transport, local trauma either in the womb or shortly after birth, or a benign genetic disorder. Other causes are inflammation, freckle (diffuse nevus) of the iris, and Horner’s syndrome.

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