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

-An undergraduate native the UK

August 29, 2013

This is a good question! native a first pass we would certainly conclude that blue eyes and blonde hair space recessive. Regrettably we’d it is in 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 way you number out if a characteristics is dominant or recessive is girlfriend look for patterns. Together I’ll explain in more detail later, if a characteristics is recessive, climate it can show up even if both parents don’t have that trait. Another means to tell if a properties is recessive is if both parents and all their youngsters share that very same trait.

In our instance here, both parents have brown eyes but they had a blue-eyed child. So this follows the an initial pattern meaning blue eyes are recessive. The exact same would it is in true because that blonde hair. Both parents had brown hair however they had a boy with blonde hair.

As ns said, though, wednesday be wrong. It transforms out the eye color and also most hair color are means too complicated to be straightforward dominant/recessive traits.

Surprisingly, blue-eyed parents sometimes have actually a brown-eyed child. Currently this no that common so we could still say that blue eye are greatly 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 friend can’t just look at a single family to number this stuff out. You need to look at lots and also lots that families and also lots and also lots that kids. Just then carry out you have actually a chance at figuring the end if a characteristics is dominant or recessive.

For example, imagine the in our case both youngsters had brown eyes. We might conclude the brown eyes are recessive because all the parents and also kids had the same eye color. However this is no at every correct. If you just look in ~ one example you can miss the best answer! 

In truth this is where the not correct ideas around attached ear lobes, rolling her tongue and also lots of various other supposedly straightforward dominant and also recessive traits came from. Researchers jumped come the wrong conclusion from studying too couple of families. 

One that the huge reasons why blonde hair and also blue eye (and every those various other traits) fail together true recessives is that they space not because of a single gene. And truly recessive traits virtually always show off a single gene.

Of food just due to the fact that a solitary gene is connected doesn’t median you’ll because that sure have a clear dominant or recessive pattern. The ABO blood type is a great example the this.

For the rest of the prize I want to emphasis on why dominant and also recessive traits monitor the fads I’ve described. And also why gift due to more than one gene deserve to disrupt the whole thing.

Why fads Work

As I pointed out it would certainly be pretty basic to prize this concern if blonde hair and blue eye were as result of a solitary gene. Let’s watch why.

For this, we’ll emphasis on a dominant trait that really is because of a single gene—the capability to taste Phenylthiocarbamide or PTC. PTC is a bitterness tasting chemical similar to one found in broccoli and also brussel sprouts that three out of every four human being can taste.

With this trait, two parents who can’t taste PTC rarely have a boy that can taste it. And sometimes parental who deserve to taste PTC have actually a boy that can’t taste it. This fits our patterns meaning that the ability to taste PTC is a leading trait (and not being able come taste is a recessive one). Let’s dig a little deeper to check out why.

There space two versions of the gene the determines if you have the right to taste PTC—T and also t. The T version lets you taste PTC and also the t version does not.

Another surname for this two different versions the the same gene are alleles. Even if it is or no a person can taste PTC depends on which alleles that the gene they have. 

If this is every there to be then it would be an extremely hard to view why tasters also had nontasters. To know that, we need to know that human being have two copies of every gene. They obtain one copy from your mom and also one from their 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 stop true if they obtain two duplicates of the t allele. In this situation they will be tt and so not have the ability to taste PTC.

However, if a human being gets one copy of the T allele, and also one that the t allele, it transforms out they can still taste PTC. This way that tasting PTC (the T allele) is dominant since if you gain a solitary copy native either parental you display that variation of the trait. In various other words, this carriers have the right to taste PTC.

The opposite is true for not being able come taste PTC (the t allele). That is recessive, as it will be concealed whenever a person has actually a T copy the the gene. For this reason the only combination that will reason a human being to NOT be able to taste PTC is once they have actually two copies of the t allele (tt). 

Now we are almost ready to define why nontaster parents rarely have a taster child and why taster parental sometimes have actually a boy that can not taste PTC. The last piece of the puzzle we require is the each parental passes simply one of your alleles to his or she child. And that allele is favored at random.

Now us are prepared to view where the trends come from. We’ll execute this by looking at three different scenarios.

First, stop imagine two nontaster parents. Right here is what this could look prefer genetically:


First off, you can see that both parents space tt. This is the an interpretation of a nontaster. 

Remember each kid will acquire one allele native mom and one indigenous dad. Due to the fact that the parents can only pass a t, every the youngsters are guaranteed to it is in tt. None of them will be able to taste PTC. This is why parents v a recessive trait have youngsters with recessive traits too*.

Now let’s look at 2 tasters who happen to be TT:


Again, castle only have one allele they deserve to pass down…in this instance a T. This method all their kids will be TT and so will be able to taste PTC.

Finally, stop look at two tasters that bring a covert nontaster allele. Together you deserve to see, both parents are Tt:


In this case, if both mom and also dad take place to pass their t to your child, then the child will be tt. That boy will not have the ability to taste PTC.

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

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

*Even this no 100% because that PTC. Sometimes parents who can’t taste PTC have actually a boy that can. There are a variety of ways this have the right to happen yet most of the time, this parents would have a kid who can not taste PTC.

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Different Patterns

We won’t have actually time to get in it here, yet if there are multiple genes involved, these trends can breakdown pretty quickly. Click below for an example of this with eye color and here because that one with blood type.

When us look at lots of families for eye color, we view a sample that type of looks like blue eyes space recessive and also brown room dominant. But we additionally see the more than occasional blue eyed parents having actually a brown eyed child. And also we aren’t even throwing in green and also hazel and also everything else!

Hair color is an even bigger mess. If we look at enough families, we tend to see several blending. A dark haired parent and a irradiate haired parental will often have a child with a color in between. Black color + Blonde = Brown!

So all in every the answer to your inquiry is neither! Blonde hair, brown hair, blue eye, browns eyes …none of those properties are leading or recessive together they space not because of a solitary gene. Which in a lot of of ways is a great thing since multi-gene traits permit for every one of the wonderful variation us see roughly us!