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Biology - eye colour genetics

Expert: Dana Krempels, Ph.D. - 11/4/2009

Question
Hi there. I have a question about eye colour. I have blue eyes but my father has brown eyes and my mother has green ones. My sister has brown eyes also. My dad's mother was an albino with colourless eyes and his father had brown eyes. My mum's mum had greenish/hazel eyes and her father blue. I was wondering if the fact that I have blue eyes (and not brown which I thought to be dominant) may mean I have inherited my father's other gene which doesn't show, which I would presume to be the albino one, but this is from my vague knowledge of genetics from high school. I'd really appreciate an answer as it really interests me. Thanks!

Answer
Dear Beck,

Human hair, eye, and skin color are very complex and difficult to predict, because each of these traits is controlled by more than one gene. It's not really a matter of a child taking after the father or mother's side. Genes don't work that way. What matters is which parent has the dominant versions of the various genes that affect the traits in question, because these are the ones most likely to be expressed by the child--though not always.

To fully explain this, I need to give you a quick overview of basic genetics. Here goes!

Every animal (including human) carries two copies of every gene. Scientists now estimate that a human has about 30,000 genes in his/her genome, and every human has two copies of that genome: one from mom, and one from dad. The two versions of each gene (called *alleles*) may be the same in a single person, or they may be different. What does that mean?

A brief example. Let's say that there's a human gene that codes for the shape of the forehead hairline. There are two versions of the gene. One, which we'll call "W", codes for a small "V" of hair to point down onto the forehead (Widow's Peak). The other version, which we'll call "w", codes for a straight hairline. In this case, the W allele of the gene masks the expression of the w allele. The W is said to be *dominant*, and the w is said to be *recessive*. So if every person has two copies of this gene, then the possible combinations are:

WW - Widow's peak
Ww - Widow's peak
ww - straight

Human hair, skin and eye color are not that simple. Instead of being controlled by only one gene, these traits are each controlled by *several* different genes, each with two or more versions (alleles).
This means that the different versions can combine and interact in unpredictable ways to produce a wide range of phenotypes (physical appearance).

A trait that is controlled by several genes is called a POLYGENIC TRAIT. A polygenic trait is the expression of a single phenotypic trait that is affected by the action of more than one gene.

There are too many examples to list, since most traits are--at least to some degree--polygenic. But human hair color, eye color, and skin color are among them.

One cute, easy-to-see example of a polygenic trait is the inheritance of fruit color in bell peppers, and it is a bit analogous to the human traits just named. There are at least three genes involved here, which we'll abbreviate as:

* Y - timing of chlorophyll elimination (Y - early; y - normal)
* R - color of carotenoids (R - red; r - yellow)
* C - regulation of carotenoid deposition
  (C - normal; c1, c2 - lowered concentration)

(The capital letters indicate the dominant alleles; the lower case indicate various versions of recessive alleles.)

This leads to a few possible genotypes producing interesting phenotypes:
       o Y- rr c1c2 - pale yellow
       o Y- rr Cc2 - darker yellow
       o yy rr CC - green
       o Y- R- CC - red
       o yy Rr CC - purple
       o Y- Rr Cc2 - pale yellow

You can see what these look like here:

http://www.bio.miami.edu/dana/pix/bellpeppers.jpg

See? It is a little bit like human color, but in this case there are only *three* genes involved. Imagine how complicated things get when there are more than three genes, as there are in human hair, eye, and skin color!

The more genes involved in the expression/appearance of a trait, the more possible variations there are, and the more difficult (perhaps impossible) it becomes to guess what a baby will look like, especially if you don't know the exact genetics of the parents. (Knowing the grandparents' phenotypes can help, but usually not very much.)

Hair color is a result of interaction between several genes that not only control the *color* of the hair pigmentation (one gene controls the expression of BROWN *eumelanin* pigment and a different gene controls expression of RED *phaeomelanin* pigment), but also *how much* pigment is deposited in the hair shaft. The darker the hair, the greater the melanin deposition, but one can't really predict how dark a baby's hair will be, since s/he may inherit a wide variety of "darkness level" genes from both parents, and they can recombine in various ways to produce hair that ranges in color from very light to very dark.

If a person expresses both the eumelanin (brown) and phaeomelanin (red) genes, the hair will be reddish brown. Dark to light brown hair with no trace of red occurs when only eumelanin is expressed, but in varying concentrations. Blonde hair with no trace of red occurs when there is weak eumelanin expression and *no* phaeomelanin. Red hair occurs when there is strong expression of phaeomelanin and weak expression of eumelanin. Not all people express both genes, but in dark-haired people that do express both, you can sometimes see a reddish sheen in the hair in certain light. But the darker eumelanin pigment often makes it difficult to see the red pigment, if it's present.

So you can see this is pretty complicated, even with just a few genes.

That said, red (auburn) hair is usually considered recessive to the other hair colors, as its expression can be masked if there is dark deposition of eumelanin. Again, it's nearly impossible to predict what the combination of genes in your baby will produce, and hair color can also *change* with age!

Light colored eyes (blue, green, hazel, grey, etc.) are usually considered recessive to dark-colored eyes. But this trait is controlled by at least five different genes. There are genes that control (1) whether or not melanin is deposited in the iris (the dominant B allele codes for brown, and the recessive b allele, coding for no melanin, will result in pale irises. These will be blue in the absence of other pigments), (2) the *amount* of pigment deposited (several genes that can combine to generate eyes that are very dark, almost black to relatively light brown), as well as (3) overlying carotenoid pigments that can change a blue iris to green, aqua, grey, or any number of variations.

And to make things even more complicated, eye color--like hair color--can change with age.

Still, one can predict, to some degree, whether a child will have light-colored or brown eyes. The allele coding for light eyes (i.e., lack of melanin in the iris) is recessive to the allele coding for dark eyes (i.e., melanin deposited in the iris). We usually abbreviate the light eye allele as "b" and the dark-eye allele as "B"--they are different versions of the same gene.

For a person to have light eyes, s/he must inherit two copies of the b allele (genotype bb). A person needs only one copy of the B gene to have dark (brown) eyes, so can be either BB or Bb.

Skin color is probably the most complex of all the traits. Freckles are apparently controlled by only one gene, and freckles are considered dominant to non-freckles. But various factors during development can affect this, and exposure to sunlight can also determine the level of freckling that is expressed.

The shade of the skin in humans may be controlled by several genes, each with several alleles, and this makes the prediction of skin tone in a baby a nearly impossible task.

Now, what does this mean to *your* situation?

Your eyes are blue because you inherited the recessive form of the iris "base color" allele from your father, which is blue. It's not possible to tell whether your paternal grandmother's genotype with respect to eye color because of the albinism, which is controlled by a completely separate gene locus. Albinism (a recessive allele that can have several different forms, any of which interferes in one or more of the many enzymatic steps needed to convert the amino acid tyrosine to melanin) prevents the expression of any melanin-related traits, but it is not the melanin trait itself. Such a gene, which exerts "control" over the expression of other gene loci (in this case, any of the genes controlling the type of melanin or their deposition in tissues) is called an *epistatic* gene, and the genes whose expression it masks are said to be *hypostatic*.

In short, your father carries a recessive blue-eye allele that he received from either his father or mother (his father could also be heterozygous for eye color, but still have brown eyes).

The other interesting thing is that since your father is a non-expressing carrier of the albinism mutation expressed by your mother, there's a 50% chance that he passed that on to you or any of your siblings. Your children won't express it unless you marry someone who carries the same version of the albinism mutation, though.

Hope this helps clear things up. :)

Dana

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