Why Are Brown Eyes Dominant?
- Pieter Maas
Why are our kids’ eyes different colours? – Let’s look at why a blue-eyed parent (dad) and a brown-eyed parent (mum) and can have brown, green, and blue-eyed children. For gene 1, OCA2, there are two possibilities: brown or blue. The brown version of gene 1 is dominant over the blue one. Dominant means that if at least 1 of your two copies is brown (Bb), then you will have brown eyes. Geneticists represent the different versions of the eye colour gene as B for brown and b for blue (the capital letter is the dominant, the lowercase, recessive).
So brown eyes are either Bb or BB and blue eyes are bb. For gene 2, there are two possibilities, green or blue. Green is dominant over blue. Green eyes can be GG, or Gb, while blue eyes are bb. Brown is dominant over green, so if you have a B version of gene 1 and a G version of gene 2, you will have brown eyes.
The possible gene combinations that can give you brown, green, or blue eyes are shown in the chart. Back to the green or blue-eyed children. Dad can only be bb bb as he has blue eyes. Since mum has brown eyes, she could have any of six different possibilities.
Why are brown eyes considered dominant?
For example, if a mother has two blue-eye genes and a father has two brown-eye genes then each child inherits a brown-eye gene from their father and a blue-eye gene from their mother. The brown-eye gene is dominant and overrides the blue-eye gene, so all the children have brown eyes.
Are brown eyes actually dominant?
The Genetics of Eye Color Download the PDF version of Biotech Basics: Genetics of Eye Color Countless students have been taught that a single gene controls eye color, with the allele for brown eyes being dominant over blue. Scientists now realize such a model is overly simplistic and incorrect. What you need to know:
DNA provides the set of recipes, or genes, used by cells to carry out daily functions and interact with the environment. Eye color was traditionally described as a single gene trait, with brown eyes being dominant over blue eyes. Today, scientists have discovered that at least eight genes influence the final color of eyes. The genes control the amount of melanin inside specialized cells of the iris. One gene, OCA2, controls nearly three-fourths of the blue-brown color spectrum. However, other genes can override the OCA2 instruction, albeit rarely. This multifactorial model for eye color explains most of the genetic factors that influence eye color.
Introduction In 1907, Charles and Gertrude Davenport developed a model for the genetics of eye color. They suggested that brown eye color is always dominant over blue eye color. This would mean that two blue-eyed parents would always produce blue-eyed children, never ones with brown eyes.
- For most of the past 100 years, this version of eye color genetics has been taught in classrooms around the world.
- It’s one of the few genetic concepts that adults often recall from their high school or college biology classes.
- Unfortunately, this model is overly simplistic and incorrect – eye color is actually controlled by several genes.
Additionally, many of the genes involved in eye color also influence skin and hair tones. In this edition of Biotech Basics, we’ll explore the science behind pigmentation and discuss the genetics of eye color. In a future edition, we’ll discuss genetic factors that contribute to skin and hair color.
- A primer on pigmentation The color of human eyes, skin and hair is primarily controlled by the amount and type of a pigment called melanin.
- Specialized cells known as melanocytes produce the melanin, storing it in intracellular compartments known as melanosomes.
- The overall number of melanocytes is roughly equivalent for all people, however the level of melanin inside each melanosome and the number of melanosomes inside a melanocyte varies.
The total amount of melanin is what determines the range of hair, eye and skin colors. There are a number of genes involved in the production, processing and transport of melanin. Some genes play major roles while others contribute only slightly. To date, scientists have identified over 150 different genes that influence skin, hair and eye pigmentation (an updated list is available at ).
A number of these genes have been identified from studying genetic disorders in humans. Others were discovered through comparative genomic studies of coat color in mice and pigmentation patterns in fish. (A previous Biotech101 article that provides an overview of comparative genomics can be found,) figure one Eye color genes In humans, eye color is determined by the amount of light that reflects off the iris, a muscular structure that controls how much light enters the eye.
The range in eye color, from blue to hazel to brown (see figure one), depends on the level of melanin pigment stored in the melanosome “packets” in the melanocytes of the iris. Blue eyes contain minimal amounts of pigment within a small number of melanosomes. Irises from green–hazel eyes show moderate pigment levels and melanosome number, while brown eyes are the result of high melanin levels stored across many melanosomes (see figure two, left).
- To date, eight genes have been identified which impact eye color.
- The OCA2 gene, located on chromosome 15, appears to play a major role in controlling the brown/blue color spectrum.
- OCA2 produces a protein called P-protein that is involved in the formation and processing of melanin.
- Individuals with OCA2 mutations that prevent P-protein from being produced are born with a form of albinism.
These individuals have very light colored hair, eyes and skin. Non-disease-causing OCA2 variants (alleles) have also been identified. These alleles alter P-protein levels by controlling the amount of OCA2 RNA that is generated. The allele that results in high levels of P-protein is linked to brown eyes.
Another allele, associated with blue eye color, dramatically reduces the P-protein concentration. On the surface, this sounds like the dominant/recessive eye color model that has been taught in biology classes for decades. However, while about three-fourths of eye color variation can be explained by genetic changes in and around this gene, OCA2 is not the only influence on color.
A recent study that compared eye color to OCA2 status showed that 62 percent of individuals with two copies of the blue-eyed OCA2 allele, as well as 7.5 percent of the individuals who had the brown-eyed OCA2 alleles, had blue eyes. A number of other genes (such as TYRP1, ASIP and ALC42A5 ) also function in the melanin pathway and shift the total amount of melanin present in the iris.
- The combined efforts of these genes may boost melanin levels to produce hazel or brown eyes, or reduce total melanin resulting in blue eyes.
- This explains how two parents with blue eyes can have green- or brown-eyed children (an impossible situation under the Davenport single gene model) – the combination of color alleles received by the child resulted in a greater amount of melanin than either parent individually possessed.
As a side note, while there is a wide variability in eye color, colors other than brown only exist among individuals of European descent. African and Asian populations are typically brown-eyed. In 2008 a team of researchers studying the OCA2 gene published results demonstrating that the allele associated with blue eyes occurred only within the last 6,000 – 10,000 years within the European population.
- Pigmentation research at HudsonAlpha Dr.
- Greg Barsh, a physician-scientist who has recently joined the HudsonAlpha faculty, and his lab study key aspects of cell signaling and natural variation as a means to better understand, diagnose and treat human diseases.
- In particular, his work has focused on pigmentation disorders.
He has explored mutations that affect easily observable traits—such as variation in eye, hair or skin colors—as a signpost for more complex processes such as diabetes, obesity, neurodegeneration and melanoma, the most serious form of skin cancer. – Dr.
What is the most dominant eye color in the world?
What is the most common eye color? – About 10,000 years ago, everyone in the world had brown eyes. Scientists believe that the first blue-eyed person had a genetic mutation that caused the body to produce less melanin. Today, about half of the people in the United States have brown eyes.
Amber, which some people describe as copper, gold or very light brown. Blue or gray, which occurs when someone has no pigment (melanin) in the front layer of the iris. Around 1 in 4 people in the U.S. have blue eyes. Brown, which is the most common eye color in the world. Green, which is the least common eye color. Only 9% of people in the United States have green eyes. Hazel, a combination of brown and green. Hazel eyes may also have flecks or spots of green or brown. In the U.S., about 18% of people have hazel eyes.
Is blonde hair dominant or recessive?
How can two brown-haired parents have a blond child? The genetics of hair color is still a bit of a mystery, but we do know a few things. I’ll give you a mostly-true answer first, and then dive into the ugly details if you’re still with me. If two brunette parents have a blond child, that means they had to have instructions for making blond hair hidden in their DNA.
- How could blond-making DNA be hiding in every cell of a brunette person’s body without them growing any blond hair? The answer lies in understanding ‘dominant’ and ‘recessive’ features.
- You probably know that DNA is organized into small pieces called genes, which help control how our bodies are built.
And your genes can come in different versions, called alleles, Let’s pretend that there’s just one gene that controls hair color, with a ‘brown’ and a ‘blond’ allele. Seems simple enough, right? You have the blond allele, you have blond hair. Brown allele, brown hair.
- But there’s a catch – you have two copies of each of your genes.
- You got one from each of your parents.
- What happens if you got a ‘brown’ allele from your mother, and a ‘blond’ one from your father? It turns out that brown hair is dominant,
- That means that even if only one of your two alleles is for brown hair, your hair will be brown.
The blond allele is recessive, and gets covered up. If two brunette parents have a blond child, they had to have instructions for making blond hair hidden in their DNA. You can think of recessive alleles as t-shirts, and dominant ones as jackets. If you wear one of each, only the jacket will be visible.
In the same way, features created by recessive alleles only show up if there isn’t a dominant allele around. Since you have two copies of each gene, that means the only way to have a recessive feature like blond hair is for both of them to be the recessive allele. Having one of each will leave your hair brown, even though you’ll secretly be carrying the blond allele in every cell of your body.
This is what both of our brown-haired parents had going on. They were secretly wearing blond shirts under their brunette jackets, and when it came time for each of them to pass down one item of clothing to their kid, they both gave the shirt! Image by G. Riesen With this analogy, you can also see how two blond parents should never have a brown-haired child. Since they’re both blond, they must each have two blond t-shirts and no brunette jacket. No matter which genes they pass down, their kid should end up blond.
- That being said, don’t dust off your family tree just yet – remember I said there were ugly details? For the most part the idea of a single brown/blond gene matches how hair colors are really passed down.
- If you add in a second gene with a recessive allele that makes red hair you’ve got a decent model of most hair colors and a nice story to go with it.
But as usual when it comes to nice stories, reality isn’t quite so simple. To get to the root of things, we’ll need to learn about the cells that color our hair: melanocytes. Melanocytes make the pigment that gives our body color: melanin, There are two types of melanin in your hair, eumelanin and pheomelanin, Image by G. Riesen So what controls how much of each type of melanin your melanocytes make? As you might imagine, lots of different genes can impact how these cells work. In fact, dozens of genes involved in everything from building your hair’s roots to repairing DNA have been found to affect hair color.
Unlike in our mostly-true model, there isn’t just one gene that controls melanin levels. Skin and hair color are both what we call polygenic features: they depend on a bunch of different genes working together, with more still being discovered. To make things worse, two people can have all of the same genes but still have different hair colors! Hormones can change the way that genes work, which often causes blond hair to get darker as children get older.
The Truth About Brown Eyes
With more age, melanocytes can start to die off, leading to inkless, gray/white hair. Stress, sunlight, smoking, and eating poorly can also cause hair color to fade. It’s a bit like those little printers are breaking, or simply running out of ink! I bet there’s a whole lot of money to be made in hair, if only someone could figure out how to keep melanocytes healthy longer As you can see, there’s a lot going on here.
Hair color is just one feature that comes from the combined efforts of all of a person’s genes plus their environment. We can identify some big-picture patterns like the dominance of brown hair over blond, but we need more detailed models to explain colors like strawberry blond or how color changes with age.
Lots of scientists are still working to build better models of how people’s genes create their hair colors and other features. If you really want to know exactly how it works, you’ll have to join them and do your own research – maybe you’ll discover how to un-gray hair, or even reverse aging itself! These kinds of mysteries show up at the bottom of any question, if you dig far enough.
Why are brown eyes more trustworthy?
The latest Czech research shows that eye color doesn’t have as great an impact on a person’s perceived trustworthiness as certain facial features. Anyone who’s been told they have “one of those faces” may want to take a look in the mirror to find out why.
- It could be because you have brown eyes, but if the latest research is true, those soothing, chocolate-y pools of yours have nothing to do with it.
- According to a new study released today from Charles University in Prague, while brown-eyed people may be seen as more trustworthy than blue-eyed ones, it has more to do with their face shape than their actual eye color.
Fun fact: Previous research on the subject has shown that blue-eyed babies are typically more inhibited, shy, socially wary, and timid than brown-eyed babies.
What causes dominant eyes?
How does a dominant eye occur? – At some point in your life, you’ve probably heard someone referring to a “dominant eye”, whether that’s in a sporting setting, optician’s context, or another situation entirely. Contrary to popular belief, your dominant eye doesn’t always have better vision than your non-dominant eye.
Essentially, it just means that this particular eye relays information more accurately to your brain’s visual cortex than the other eye. The cortex is responsible for processing visual information. It’s made up of bands of neurons which respond preferentially to input from one eye or the other, It’s this preference which determines the eyes’ dominance, not the visual acuity of each eye.
The dominant eye simply has more neural connections to the brain than the non-dominant eye. Almost everyone has a dominant eye, even if the difference between the two eyes doesn’t feel that stark. All About Vision suggests that, if it appears there is no dominance at all, then it’s likely that each eye is dominant for particular visual tasks, taking it in turns to function more powerfully.
Are brown eyes more dominant than green?
Genetics and Eye Color – Eye color is determined by multiple variations of genes that are in charge of the production and distribution of melanin, pheomelanin, and eumelanin. The main genes influencing eye color are called OCA2 and HERC2. Both are located on human chromosome 15.
- Each gene has two different versions (alleles).
- You inherit one from the mother and one from the father.
- If the two alleles of a specific gene are different ( heterozygous ), the trait that is dominant is expressed (shown).
- The trait that is hidden is called recessive.
- If a trait is recessive, like blue eyes, it usually only appears when the alleles are the same ( homozygous ).
Brown eye color is a dominant trait and blue eye color is a recessive trait. Green eye color is a mix of both. Green is recessive to brown but dominant to blue.
Are brown-eyed men more dominant?
There’s something in the faces of brown-eyed white men that makes them come off as more dominant than their blue-eyed peers, a new study suggests. And it isn’t their eye color. Czech researchers asked a group of 62 peopleto look at photos of 80 faces – 40 men and 40 women – and rate them for dominance.
Then the investigators Photoshopped the faces so the brown eyes were replaced with blue ones and vice versa. A separate group of participants rated the altered images for dominance. The results were the same in both cases: Faces of brown-eyed men were rated more dominant than those of blue-eyed men, even when their eyes weren’t brown.
The effect, which didn’t hold for female faces, may have something to do with the shape of brown-eyed men’s faces, said study researcher Karel Kleisner of Charles University in Prague. On average, brown-eyed men had broader chins and mouths, larger noses, more closely spaced eyes and larger eyebrows than blue-eyed men.
More mysterious is why eye color would be so closely associated with facial type, the researchers say. “It’s really quite a surprising finding and certainly not one I would necessarily expected,” said psychologist Benedict Jones of the University of Aberdeen in Scotland, who was not involved in the study.
Jones said it would be important for other researchers to investigate the link to be sure it isn’t “an idiosyncrasy of this particular sample of face images.” Researchers believe blue eyes didn’t exist until sometime between 6,000 and 10,000 years ago, when a genetic mutation emerged that reduced production of the brown pigment melanin in the iris.
- Before that, everyone had brown eyes.
- There are now half a dozen different genes that influence whether someone will have blue or brown eyes.
- One possibility, according to the researchers, is that these same genes that confer eye color have other effects on the body or are in close proximity to other genes that do, such as those that regulate the production of testosterone,
If two genes are close together on a chromosome, they are more likely to be found together in the same individual. Kleiner said the “most elegant” explanation would be that children are treated differently depending on their eye color, and that this treatment leaves a lasting effect on their bodies.
“It is possible that subjects with blue eyes are treated as for a longer period than brown-eyed children,” he and his colleagues write in the July issue of the journal Personality and Individual Differences. Previous studies have found that blue-eyed boys of preschool age were more inhibited than boys with brown eyes.
Although the effect seemed to go away within a few years, a person’s social standing can alter their hormonal balance, which could have lasting effects on face shape, Kleisner said. Kleisner said it’s conceivable that women’s sexual choices could be enhancing the link between eye color and face shape.