Dominant and Recessive Genes: The Secret of Light Eyes

FROM THE LECTURE SERIES: Understanding the Misconceptions of Science

By Don Lincoln, Ph.D., Fermi National Accelerator Laboratory (Fermilab)

Most human characteristics are governed by more than one gene. Learn more about the single gene trait, important genes dealing with eye color, and why brown eyes are dominant and light-colored recessive?

3D illustration of an eye showing blue color.
Eye color can be either brown or blue. The dominant trait is found in brown eyes and recessive in blue eyes. (Image: adike/Shutterstock)

Huntington’s Chorea

Huntington’s chorea is a disease that is deadly and has a dominant gene. Someone with this disease can give it to their offspring with a tragic outcome. Human characteristics like height, intelligence, or athletic ability aren’t a matter of just a single gene because there aren’t just short and tall people but, with cases like dwarfism and Marfans syndrome aside, men from North America range from about 5 feet 2 inches to about 6 feet 5 inches. If there was a single gene, two classes of people—short and tall—would be expected. Researchers have identified over 700 genes that go into determining a person’s height, covering things like the length of the bones and the thickness of tissues between joints.

The Single Gene Trait

There is one genetic variation, a human trait that has a single gene governing it, and that is eye color which is blue or brown. Brown eyes are dominant and blue eyes recessive. Following the logic of yellow and green peas and making a Punnett square, it should be impossible for two blue-eyed parents to have a brown-eyed child but it would be possible for brown-eyed parents to have a blue-eyed child if both parents were a genetic mixture of one blue-eye and one brown-eye gene. In the single-gene paradigm, two blue-eyed parents have only blue-eye genes and can, therefore, produce only blue-eyed children. However, eyes are not just blue or brown but some people in the Asian population have eyes so dark that look almost black. There are blue-eyed Scandinavians, and groups with gray, green, violet, hazel, and a myriad of colors.

Learn more about the human characteristics that aren’t governed by a single gene.

Complex Interaction of Genes

There are two important genes dealing with eye color, located near one another on one of the chromosomes in the human genome, and there are 10 other genes that play a minor role. In addition, there are complex interactions between the genes themselves, so it is possible for two blue-eyed parents to produce a brown-eyed child but is rare. If there is a population of mixed genes, then the dominant population will win out. Taking the idea that brown eyes are dominant in Mendelian sense, then blue eyes would be destined for extinction in the long term, which isn’t true.

Big-B Little-b

To simplify things, let us call brown eyes big-B and non-brown eyes little-b. A person has brown eyes if they are (big-B) (big-B) or (big-B) (little-b) and blue or green if they are (little-b) (little-b) because brown (big-B) is dominant over blue and green (little-b). Imagine starting out with eleven (little-b) (little-b) people and one (big-B) (little-b) person. The (big-B) (little-b) person has four children with one of the (little-b) (little-b) people and each (little-b) (little-b) couple also has four children.

Using the old Mendelian genetics, there are 20 (little-b) (little-b) people from five (little-b) (little-b) couples and two (big-B) (little-b) and two (little-b) (little-b) from the mixed couple. So there are two people with brown eyes and 22 with blue or green. Brown did not become more common. Whether a gene is dominant or not does not affect how common a trait is.

Learn more about the complexities of learning styles and IQ scores.

The Melanin Factor

Illustration explaining the anatomy of melanin.
The substance that darkens the skin is called melanin. Melanin blocks the damaging ultraviolet rays, and some people have more melanin than others. (Image: gritsalak karalak/Shutterstock)

In Scandinavia, there are a lot of blue-eyed people, perhaps because eye color is closely connected with skin color from a genetic point of view. When humans originally evolved, it was under the hot African sun and there was enormous evolutionary pressure to evolve darker skin and eyes to keep everyone safe. Human ancestors were, without a doubt, dark, and the substance that darkens skin is called melanin.

However, things changed when people wandered. It took tens of thousands of years, but human ancestors wandered northward, crossing into the Middle East and spreading out from there. One small group headed northwards to end up in northern Europe, where the sun isn’t as bright. In genetic terms, there was no pressure from the environment to keep the skin and eyes dark but rather to lighten the skin. Humans need vitamin D, to help make bones stronger, and they make this by absorbing sunlight.

The Secret of Light Eyes

Melanin blocks the damaging ultraviolet rays, and lighter-skinned people absorb sunlight better. Up in the north, with less sunlight, there is an evolutionary pressure to lighten the skin. This is not true with eyes, as they do not make vitamin D. But the biochemical process that darkens skin is connected to eye color.

By ‘lightened,’ it did not mean everyone turned into a blue-eyed person but that the eyes lightened, so there were more green, hazel, and violet eyes. What made Scandinavian eyes blue is not completely understood but it is not pressure from the environment.

The Logic of Dominance

One important point is that even if brown eyes are dominant, and if there is no pool of brown-eyed people to reproduce with, the blue eyes will continue to persist. The bottom line is that even though blue eyes are recessive and brown eyes are dominant, it is likely that there will be blue-eyed people on the Earth for many generations. There are other examples of dominant traits that are rare, such as dimples.

This is a transcript from the video series Understanding the Misconceptions of Science. Watch it now, on Wondrium.

Six-Toed Cats

Having six fingers is rare but also dominant. In Ernest Hemingway’s house in Key West, Florida, there is a house full of cats with six toes. This is because Hemingway had a cat by the name of Snow White with six toes, a gift from a sea captain, and sailors favored cats with extra toes, thinking they were good luck.

Close-up of a cat's paw with six toes at Hemingway's house in Key West.
Six toes is a dominant trait and cats with such toes are found in Key West. The original cat with six toes was favored because of the belief that those are lucky. (Image: RaksyBH/Shutterstock)

The entire island of Key West has many six-toed cats. It is a small island and many current feline residents are descendants of Snow White. Because extra toes are dominant, this particular mutation is common here, but not in the general population. Whether a trait is rare or not, depends on whether it is dominant and what the surrounding population looks like.

Rare Dimples

A dominant trait will only become common if there is a selection for it. Dimples are dominant, but they don’t give the person who has them any survival advantage. Dimples do not make it easier to survive long periods of time where food is scarce. That is why they stay rare, even though they are dominant.

Common Questions About Genetics

Q: Who is affected by Huntington’s disease?

Huntington’s chorea is deadly and has a dominant gene. Someone with this disease can give it to their offspring.

Q: What does melanin do in the body?

The substance called melanin in the body darkens the skin.

Q: Why did Hemingway’s cats have six toes?

Hemingway had a cat by the name of Snow White with six toes, a gift from a sea captain, who apparently thought those as good luck.

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