What parent passes down eye color?
What are Dominant and Recessive?
The terms dominant and recessive describe the inheritance patterns of certain traits. That is, they describe how likely it is for a certain phenotype to pass from parent offspring.
Sexually reproducing species, including people and other animals, have two copies of each gene. The two copies, called alleles, can be slightly different from each other. The differences can cause variations in the protein that’s produced, or they can change protein expression: when, where, and how much protein is made. Proteins affect traits, so variations in protein activity or expression can produce different phenotypes.
A dominant allele produces a dominant phenotype in individuals who have one copy of the allele, which can come from just one parent. For a recessive allele to produce a recessive phenotype, the individual must have two copies, one from each parent. An individual with one dominant and one recessive allele for a gene will have the dominant phenotype. They are generally considered “carriers” of the recessive allele: the recessive allele is there, but the recessive phenotype is not.
The terms are confusing and often misleading
Dominant and recessive inheritance are useful concepts when it comes to predicting the probability of an individual inheriting certain phenotypes, especially genetic disorders. But the terms can be confusing when it comes to understanding how a gene specifies a trait. This confusion comes about in part because people observed dominant and recessive inheritance patterns before anyone knew anything about DNA and genes, or how genes code for proteins that specify traits.
The critical point to understand is that there is no universal mechanism by which dominant and recessive alleles act. Dominant alleles do not physically “dominate” or “repress” recessive alleles. Whether an allele is dominant or recessive depends on the particulars of the proteins they code for.
The terms can also be subjective, which adds to the confusion. The same allele can be considered dominant or recessive, depending on how you look at it. The sickle-cell allele, described below, is a great example.
The sickle-cell allele
Sickle-cell disease is an inherited condition that causes pain and damage to organs and muscles. Instead of having flattened, round red blood cells, people with the disease have stiff, sickle-shaped cells. The long, pointy blood cells get caught in capillaries, where they block blood flow. Muscle and organ cells don’t get enough oxygen and nutrients, and they begin to die.
The disease has a recessive pattern of inheritance: only individuals with two copies of the sickle-cell allele have the disease. People with just one copy are healthy.
In addition to causing disease, the sickle-cell allele makes people who carry it resistant to malaria, a serious illness carried by mosquitos. Malaria resistance has a dominant inheritance pattern: just one copy of the sickle cell allele is enough to protect against infection. This is the very same allele that, in a recessive inheritance pattern, causes sickle-cell disease!
Now let’s look again at the shape of the blood cells. People with two copies of the sickle-cell allele have many sickled red blood cells. People with two copies of the “normal” allele have disc-shaped red blood cells. People with one sickle-cell allele and one normal allele have a small number of sickled cells, and their cells sickle more easily under certain conditions. So we could say that red blood cell shape has a co-dominant inheritance pattern. That is, individuals with one copy of each allele have an in-between phenotype.
So is the sickle cell allele dominant, recessive, or co-dominant? It depends on how you look at it.
If we look at the proteins the two alleles code for, the picture becomes a little more clear. The affected protein is hemoglobin, the oxygen-carrying molecule that fills red blood cells. The sickle-cell allele codes for a slightly modified version of the hemoglobin protein. The modified hemoglobin protein still carries oxygen, but under low-oxygen conditions the proteins stick together.
When a person has two sickle cell alleles, all of their hemoglobin is the sticky form, and the proteins form very long, stiff fibers that distort red blood cells. When someone has one sickle-cell allele and one normal allele, only some of the hemoglobin is sticky. Non-sticky hemoglobin is made from the normal allele, and sticky hemoglobin is made from the sickle-cell allele (every cell has a copy of both alleles). The sticking-together effect is diluted, and in most cells, the proteins don’t form fibers.
The protist that causes malaria grows and reproduces in red blood cells. Just exactly how the sickle-cell allele leads to malaria resistance is complex and not completely understood. However, it appears that the parasite reproduces more slowly in blood cells that have some modified hemoglobin. And infected cells, because they easily become misshapen, are more quickly removed from circulation and destroyed.
To see more examples of how variations in genes influence traits, visit The Outcome of Mutation.
Common Myths Explained
Dominant and recessive are important concepts, but they are so often over-emphasized. After all, most traits have complex, unpredictable inheritance patterns. However, at the risk of adding even more over-emphasis, here are some more things you may want to know:
Dominant phenotypes are not always more common than recessive phenotypes
Let’s look at a typical (i.e., rare) single-gene trait:
- dominant allele + dominant allele = dominant phenotype
- dominant allele + recessive allele = dominant phenotype
- recessive allele + recessive allele = recessive phenotype
Looking at this, you might conclude that the dominant phenotype is twice as common as the recessive one. But you would probably be wrong.
Recessive alleles can be present in a population at very high frequency. Consider eye color. Eye color is influenced mainly by two genes, with smaller contributions from several others. People with light eyes tend to carry recessive alleles of the major genes; people with dark eyes tend to carry dominant alleles. In Scandinavia, most people have light eyes—the recessive alleles of these genes are much more common here than the dominant ones.
Dominant alleles are not better than recessive alleles
Mode of inheritance has nothing to do with whether an allele benefits an individual or not. Take rock pocket mice, where fur color is controlled mainly by a single gene. The gene codes for a protein that makes dark pigment. Some rock pocket mice have dark fur, and some have light fur. The dark-fur allele is dominant, and the light-fur allele is recessive.
When mice live in a habitat filled with dark rocks, dark fur is “better” because it makes the mice less visible to predators. But when mice live in a habitat filled with light rocks and sand, light fur is “better.” It’s the environment that matters, not whether the allele is dominant or recessive.
A “broken” allele can have a dominant inheritance pattern
Many genetic disorders involve “broken” genes that code for a protein that doesn’t work properly. Since one “normal” copy of the gene can often provide enough of the protein to mask the effects of the disease allele, these disorders often have a recessive inheritance pattern. But not all diseases alleles are recessive.
Keratin proteins link together to form strong fibers that strengthen hair, fingernails, skin, and other tissues throughout the body. There are several genetic disorders involving defects in keratin genes, and most of them have dominant inheritance patterns.
To see how defective keratin genes can lead to a genetic disorder, see Pachyonychia Congenita.
To see more myths explained, visit Things You May Not Know About DNA.
Funding provided by grant 51006109 from the Howard Hughes Medical Institute, Precollege Science Education Initiative for Biomedical Research.
How do we get our eye color?
Most of us learned what we know about eye color from a chart in grade-school biology. You know, the one that shows that two brown-eyed parents will likely have brown-eyed kids, and two blue-eyed parents are pretty much destined to have blue-eyed kids. It might have come with little color codes, clear-cut percentages, and neat lines of inheritance. But the story of how eye color is passed down is more complicated—and unpredictable—than we’re taught.
Why eyes look different colors
Humans get their eye color from melanin, the protective pigment that also determines skin and hair shades. Melanin is good at absorbing light, which is especially important for the iris, the function of which is to control how much brightness can enter the eyes. Once it passes through the lenses, the majority of the visible light spectrum goes to the retina, where it’s converted to electrical impulses and translated into images by the brain. The little that isn’t absorbed by the iris is reflected back, producing what we see as eye color.
Now, that color depends on the kind and density of melanin a person is born with. There are two types of the pigment: eumelanin, which produces a rich chocolate brown, and pheomelanin, which renders as amber, green, and hazel. Blue eyes, meanwhile, get their hue from having a relatively small amount of eumelanin. When the pigment is low in stock, it scatters light around the front layer of the iris, causing it to re-emerge are shorter blue wavelengths. This makes blue an example of what is called “structural color,” as opposed to brown and to some extent, green and hazel, which would be defined as a “pigment colors.” It’s in part the same reason the sky is blue—an atmospheric light trick known as the Rayleigh effect.
Green eyes are interesting because they combine light scattering and two kinds of pigment: They hold slightly higher amounts of eumelanin than blue eyes, as well as some pheomelanin. Hazel eyes come from the same combination, but they have more melanin concentrated in the outer top layer of the iris. Red and violet eyes, which are much rarer, come from a minute to complete lack of pigment. In fact, red eyes have no melanin whatsoever, so all we’re seeing is the reflection of the blood vessels. When there’s some pigment, but too little to cause wavelengths to scatter, the red and blue interact to produce a rare violet.
An imperfect circle of genes
Though we used to think eye color came from a relatively simple pattern of inheritance, in recent years scientists have found that it’s determined by many genes acting in tandem. What’s more, tiny tweaks on a gene can result in different shades in the iris. “When you have mutations in a gene, they’re not just acting in a vacuum,” says Heather Norton, a molecular anthropologist who studies the evolution of pigmentation at the University of Cincinnati. “The proteins that they produce don’t just do what they do independently.”
The two genes currently thought to be most strongly associated with human eye color are OCA2 and HERC2, which are both located on chromosome 15. OCA2, the gene we used to think to be the sole player in eye color, controls the production of the P protein and the organelles that make and transport melanin. Different mutations in the OCA2 gene ramp up or tamp down the amount of protein that’s produced in the body, changing how much melanin is sent to the irises. (If you’re wondering why some kids are born with blue eyes but end up with green or hazel ones later in life, it’s because these organelles take a while to mature and start shuttling melanin around).
The HERC2 gene, meanwhile, acts like a helicopter parent for OCA2. Different mutations in this gene act as a switch that turns OCA2 on and off and determines how much P protein it encodes.
Those are just the two genes we know about in detail so far. Newer studies have linked as many as 16 genes to eye color, all of which pair with OCA2 and HERC2 to generate a spectrum of different iris colors and patterns. With all these variations in the interaction and expression of genes, it’s hard to say for sure what a child’s eye color will be based on their parents’. “While what you might have in your HERC2 genotype matters, it also matters what you have at [other parts of chromosome 15],” Norton says. “Even though you might have two copies of the allele that’s more commonly associated with blue eye color, if you have a mutation somewhere else in your genome that does something to modulate how that P protein is produced or distributed, that’s going to influence phenotype.” What she means is, if a kid does shockingly end up with brown eyes, there’s no need to flip out and reach for the paternity test. It’s just the rich tapestry of genes.
Norton notes that most of what we know about the complicated genetics of eye color, we know through genome-wide association studies (GWAS), which track visible traits in subjects with varying DNA profiles. But she also points out that there are huge gaps in the range of populations we’ve documented to figure out how eye color is genetically influenced. “Given that most of what we know about how these genetics have been done in studies of Europeans, when you think about some of these genetic interactions, there may be mutations that influence eye color, skin color, or hair color that are more common in other parts of the world,” Norton says. “We don’t know about them because we don’t look.”
There are several research groups around the world trying to flip this bias by conducting GWAS studies in Latin American and South African populations; some have even found novel gene segments affecting skin pigmentation in different communities. One day, the same may be revealed about eye color.
Why choose one …
Now you might be wondering, what causes people—and sometimes really cute huskies—to have a different color iris in each eye? The condition is called heterochromia for short, and there are several kinds: partial heterochromia, where part of the iris is a different color; central heterochromia, where the inner portion of the iris is a different color than the outer ring; and complete heterochromia, where one iris is a completely different color than the other.
The vast majority of cases of congenital heterochromia (when people are born with the condition) are completely benign, but in rare cases it can occur as a symptom of disorders like Horner or Waardenburg syndromes. If heterochromia develops later in life, it’s most often the result of eye injury, head trauma, melanoma, or sporadically, glaucoma treatments. But in the majority of people it happens by random mutation, leading to one eye getting more or less melanin than it should. Try and put that in a chart.
Which parent gives you the most dominant genes?
Except for a few special cases (see below), it doesn’t really matter which parent gave you which gene. If a gene version is dominant, it will dominate whether it came from mom or dad.
Your chances of getting a dominant trait don’t depend on which parent it came from. If mom gives you a dominant brown eye version of an eye color gene, odds are you’ll end up with brown eyes. Same thing if dad passes the same gene. In neither case would you have higher odds for getting brown eyes.
Now that isn’t to say that if mom has brown eyes then all her kids will too. They could end up with the other parent’s recessive blue or green eyes. Or an eye color that neither parent has!
This is how brown-eyed parents end up with a blue-eyed child. Or how two parents who don’t have red hair have a redheaded baby.
As you can see, genetics is a complicated business. But one thing we do know. a child isn’t more likely to favor one parent over the other. Which traits you get depend on the combination of genes you get from both parents.
What I’ll do for the rest of the answer is explain a bit about how genes work. Then I’ll focus on some situations where the parents do matter. As you’ll see, this is usually when a trait is on the X chromosome.
Dominant and Recessive
Let’s say that a child has a mom with brown eyes and dad with blue eyes. In general, brown eyes are dominant to blue. That means that if you have the DNA for both brown and blue, you’ll have brown eyes.
(I’ll also note that it’s more complicated than I’m about to describe here. But the general pattern holds true, where darker eyes are more dominant than lighter ones.)
Since brown eyes are dominant, there are two possibilities for mom. She can have two copies of the brown version of an eye color gene (“BB”, as geneticists like to say). Or she could have one brown (B) and one blue (b) version of that gene, or “Bb”.
To make things easier, we will say that she is BB (both genes are the brown version). Since the dad has blue eyes, he has two copies of the recessive blue version. He is bb.
Each parent will pass one copy of their eye color gene to their child. In this case, the mom will always pass B and the dad will always pass b. This means all of their kids will be Bb and have brown eyes. Each child will show the mom’s dominant trait.
Now if we flip things around where the father has two brown versions (BB) and the mom has two blue ones (bb), the child will still end up Bb and having brown eyes. It doesn’t matter if B came from mom or dad. It only mattered that the child got a B.
I don’t want you to think that if one parent shows the dominant trait, all their children will too. They may not. Let me give another eye color example to show you what I mean.
Imagine a mom with one version of the brown and one version of the blue eye color gene. She is Bb and has brown eyes. Dad is bb and has blue eyes.
These are the same eye colors that the parents had in the first example. But the result could turn out very differently.
In this example, the kids would each have a 50% chance of having mom’s brown eyes and a 50% chance of having dad’s blue eyes. (This is because mom has a 50% chance of passing her B and a 50% chance of passing her b.) They could end up with mom’s dominant trait or dad’s recessive one. Which one is a simple matter of chance.
And if we take a Bb dad (brown eyes) and a bb mother (blue eyes), there is still a 50% chance for the child to have blue eyes. Again it didn’t matter which parent gave which gene version. What was important is that these two gene versions were involved.
Of course, eye color is harder to predict than I’m describing here. There’s more than just one gene that affects what color eyes you’ll have! But it’s still a useful example.
This is true for many, many traits besides eye color. But not all of them. Sometimes it matters whether your mom or dad has a dominant trait.
Blame (or Thank) Mom
Through our discussion so far, you may have picked up on the fact that we have two copies of our genes — one from mom and one from dad. But this isn’t true for every gene.
Whether you are a boy or a girl mostly depends on whether you have an X and a Y chromosome or two X’s. If you have an X and a Y, then you are usually a boy. If you have two X’s, then you are usually a girl.
This matters for our discussion because it means that girls (and so moms) have two copies of all the genes on the X chromosome while boys (and dads) have just one. The genes on mom’s X chromosome will dominate for her sons whether they are dominant or recessive.
Let’s look at color blindness as an example to figure out why.
Imagine that mom is colorblind. Since being colorblind is recessive, she has two copies of the color blind version of the gene (c). Geneticists say she is Xc Xc because the recessive version is on the X chromosome.
In our case, dad isn’t color blind. Since he has just one X chromosome, he has a single copy of the version of the gene that lets him tell red from green. He is XC Y. (The XC means he has the dominant version of the color vision gene on his X. The Y has no color vision gene on it and so is here as a marker.)
OK, now what happens when these two parents have sons? They are all colorblind like their mother. Her recessive trait dominates!
Let me take you through how this happened. Since the child is a boy, we know dad passed his Y (otherwise the child would be a girl). This doesn’t contribute any color vision genes.
Mom will pass one of her Xc’s to her son. The son now has an Xc and a Y. He has no dominant color vision gene version to overcome his color blind version and so is color blind like his mother. Every son will have that trait.
Colorblindness is one of a few special traits where it matters which parent a gene copy came from. For most traits it doesn’t matter. What matters is the combination of genes you get no matter the source.