Both parents have actually brown eyes and also brown hair. They have actually twins and also one has actually brown hair and blue eye while the other have actually blonde hair and blue eyes. Which attributes are dominant and how carry out you know?

-An undergraduate from the UK

August 29, 2013

This is a an excellent question! from a first pass we would conclude the blue eyes and also blonde hair are recessive. Regrettably we’d be wrong about the blonde hair (and only partly right about eye color).

You are watching: If a child has two genes for blonde hair (one from each parent), his genotype is said to be

The method you number out if a characteristics is leading or recessive is girlfriend look for patterns. As I’ll define in an ext detail later, if a properties is recessive, climate it can show up even if both parental don’t have actually that trait. Another method to tell if a trait is recessive is if both parents and also all their kids share that exact same trait.

In our instance here, both parents have actually brown eyes yet they had a blue-eyed child. For this reason this complies with the very first pattern an interpretation blue eyes room recessive. The exact same would it is in true for blonde hair. Both parents had actually brown hair but they had actually a boy with blonde hair.

As i said, though, we’d be wrong. It transforms out the eye color and most hair colors are method too complicated to be an easy dominant/recessive traits.

Surprisingly, blue-eyed parental sometimes have actually a brown-eyed child. Now this isn’t that typical so we could still say that blue eyes are mostly recessive. The exact same isn’t yes, really true for blonde hair. Blonde parents regularly have darker haired youngsters (or also redheads).

This is why girlfriend can’t simply look at a single family to figure this ingredient out. You need to look in ~ lots and lots the families and also lots and lots of kids. Just then execute you have a possibility at figuring out if a properties is dominant or recessive.

For example, imagine the in our case both kids had brown eyes. We might conclude that brown eyes are recessive due to the fact that all the parents and kids had actually the very same eye color. However this is not at every correct. If you only look in ~ one instance you deserve to miss the right answer! 

In reality this is where the dorn ideas around attached ear lobes, rolling your tongue and also lots of other supposedly basic dominant and also recessive traits came from. Scientists jumped come the dorn conclusion from examining too few families. 

One that the large reasons why blonde hair and also blue eyes (and every those various other traits) fail as true recessives is the they room not because of a single gene. And also truly recessive traits almost always indicate a single gene.

Of food just since a solitary gene is associated doesn’t typical you’ll for sure have a clear leading or recessive pattern. The ABO blood type is a great example the this.

For the rest of the answer I desire to emphasis on why dominant and recessive traits follow the patterns I’ve described. And why being due to much more than one gene have the right to disrupt the whole thing.

Why fads Work

As I mentioned it would be pretty simple to prize this concern if blonde hair and blue eye were due to a single gene. Let’s watch why.

For this, we’ll emphasis on a leading trait the really is as result of a solitary gene—the ability to taste Phenylthiocarbamide or PTC. PTC is a bitterness tasting chemical similar to one discovered in broccoli and brussel sprouts that 3 out of every four world can taste.

With this trait, two parents who can’t taste PTC rarely have actually a kid that can taste it. And sometimes parents who have the right to taste PTC have a child that can’t taste it. This fits our patterns meaning that the ability to taste PTC is a leading trait (and no being able to taste is a recessive one). Let’s dig a little deeper to see why.

There room two versions of the gene that determines if you deserve to taste PTC—T and t. The T version lets you taste PTC and also the t version does not.

Another surname for this two various versions that the very same gene room alleles. Whether or no a person deserve to taste PTC depends on which alleles that the gene castle have. 

If this is all there to be then it would certainly be an extremely hard to watch why tasters also had nontasters. To understand that, we need to understand that human being have two copies of each gene. They gain one copy from your mom and one from your dad. 

If a person gets two duplicates of the T allele (so your genotype is TT), then it is clear that they will have the ability to taste PTC. The very same idea hold true if they get two copies of the t allele. In this situation they will be tt and also so not be able to taste PTC.

However, if a person gets one copy the the T allele, and also one of the t allele, it turns out they have the right to still taste PTC. This method that tasting PTC (the T allele) is dominant due to the fact that if you acquire a solitary copy indigenous either parental you display that variation of the trait. In various other words, this carriers deserve to taste PTC.

The the contrary is true for not being able come taste PTC (the t allele). The is recessive, together it will certainly be hidden whenever a person has a T copy of the gene. Therefore the only combination that will reason a person to NOT be able to taste PTC is when they have actually two copies of the t allele (tt). 

Now we are virtually ready to describe why nontaster parents rarely have a taster child and why taster parental sometimes have actually a child that can’t taste PTC. The last item of the puzzle we require is that each parent passes simply one of your alleles come his or she child. And also that allele is favored at random.

Now us are all set to watch where the trends come from. We’ll carry out this by looking at three different scenarios.

First, let’s imagine two nontaster parents. Below is what this might look favor genetically:


First off, you have the right to see the both parents space tt. This is the an interpretation of a nontaster. 

Remember each child will get one allele native mom and also one indigenous dad. Because the parents deserve to only happen a t, all the kids are guarantee to it is in tt. None of them will have the ability to taste PTC. This is why parents through a recessive characteristics have youngsters with recessive traits too*.

Now stop look at two tasters who happen to it is in TT:


Again, they only have actually one allele they have the right to pass down…in this situation a T. This method all their kids will it is in TT and also so will have the ability to taste PTC.

Finally, stop look at two tasters that carry a covert nontaster allele. As you deserve to see, both parents room Tt:


In this case, if both mom and dad happen to pass their t to your child, climate the kid will it is in tt. That boy will not be able to taste PTC.

But the course this parents don’t need to have a child that can not taste PTC. In fact, each boy only has a 25% possibility of finishing up a nontaster. This is why we need to look at lots and also lots of families to figure out if a trait is recessive or dominant.

In one family, taster parents may have all taster children even if the parents to be carriers. This would certainly make being a taster look favor a recessive trait even though the isn’t. Again we check out the importance of looking at lots and also lots of families.

*Even this isn’t 100% for PTC. Periodically parents that can’t taste PTC have a son that can. There space a variety of ways this have the right to happen yet most that the time, this parents would have a child who can not taste PTC.

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Different Patterns

We won’t have actually time to enter it here, however if there space multiple genes involved, these trends can break down pretty quickly. Click here for an instance of this with eye color and also here for one with blood type.

When us look at too many of families for eye color, we view a sample that sort of looks favor blue eyes are recessive and brown room dominant. But we additionally see the more than sometimes blue eyed parents having a brown eyed child. And we aren’t also throwing in green and hazel and everything else!

Hair shade is an also bigger mess. If we look at enough families, we have tendency to see numerous blending. A dark haired parent and also a light haired parental will often have a kid with a color in between. Black color + Blonde = Brown!

So every in every the answer to your question is neither! Blonde hair, brown hair, blue eye, browns eye …none of those characteristics are dominant or recessive as they space not because of a solitary gene. I m sorry in a many of methods is a good thing due to the fact that multi-gene traits permit for every one of the wonderful variation we see roughly us!