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The Genetics of Taste
Each and every one of us is unique, a fact that becomes evident when studying genetics.
When learning about genetics and heredity, most students learn about genetic traits that are easily–observable such as eye, skin or hair color, the presence (or absence) of dimples or freckles, and whether a person is right- or left-handed.
But did you know that your DNA can also affect the way that you taste food?
Consider the herb cilantro: people tend to love it or hate it.
While many think cilantro is delicious, others complain that it tastes like soap. Scientists have conducted studies to determine whether our responses to cilantro are determined by our DNA, and their results suggest they may be due to DNA differences in the gene that codes for smell receptors.
The role DNA plays in taste got me thinking about one of my favorite tools for exploring genetics: PTC (Phenylthiocarbamide) paper.
PTC is a harmless chemical with a peculiar property: depending on what genes a person has, PTC can taste vastly different.
To some, PTC tastes extremely bitter. To others, PTC has no discernible taste at all. Other individuals describe the taste of PTC as mildly bitter. But why is this?
First, let me give a brief review of what DNA is and how it works to produce genetic traits.
A Brief Review of DNA and Genetics
Like all living things on the planet, our genetic information is carried in DNA. DNA is organized into discrete packets of information called genes. Each gene contains the “recipe” for making a different protein.
In many cases, the same gene can exist in different “flavors” called alleles. For instance, we all have a gene for eye color. Some of us have the allele for blue eyes while others of us carry the allele for brown eyes.** Often, when more than one allele exists for a given gene, one allele will be dominant while the other will be recessive. In our eye color example, the brown eye allele is dominant to the blue eye allele.
To make things easier, we often assign letters to denote the different alleles of a given gene. Typically, we use a capital letter to signify the dominant allele and the lowercase letter to indicate the recessive allele. (In the case of the eye color gene, we could note the dominant brown eye allele B and the recessive blue eye allele b).
We each have two copies of every gene. During conception, we received one copy from our mother and the other from our father. The two copies of our genes can be the same or different. The specific alleles we have determines the traits we express.
Reconsider the eye color gene.
We each have two copies of the eye color gene (one copy inherited from each parent).
If we have two copies of the blue eye color allele (bb), we will have blue eyes.
If we have two copies of the brown eye color allele (BB), we will have brown eyes.
But what if we have a copy of each eye color color allele (Bb)? What color eyes will we have?
Because the allele for brown eyes is dominant and the allele for blue eyes is recessive, we will have brown eyes. Even though the blue eye allele is present, its expression is masked by the dominant brown eye allele. We can still pass on the blue eye allele to our children, which is why parents with brown eyes can have children with blue eyes.
This is a straight-forward type of inheritance that follows the pattern first observed by Gregor Mendel. In honor of him, we call this Mendelian inheritance.
But not all genes “follow the rules” of Mendelian inheritance. And this brings us back to the genetics of tasting PTC.
The Genetics of Tasting PTC
The ability (or inability) to taste PTC can be traced to the different alleles that exist for the gene TAS2R38. This gene contains the instructions for making a specific taste bud receptor on the tongue.
Researchers have determined that there are two alleles for TAS2R38: a “taster” allele (T) and a “non-taster” allele (t).
The taster allele codes for a taste bud receptor shaped in a way that it binds strongly to PTC. Once this receptor binds to PTC, a signal is sent to the brain which is interpreted as an intensely bitter taste.
In contrast, the taste bud receptor produced by individuals with the non-taster allele is shaped differently and PTC is unable to bind to it. Therefore, no signal is sent to the brain to indicate a bitter taste.
Interestingly, people with a copy of both the taster and the non-taster allele (Tt) can perceive a weakly bitter taste when PTC paper is placed on their tongue. While the bitterness is not nearly as strong as those with two copies of the taster allele (TT), they can clearly identify a mild bitter taste. It’s almost as if these individuals (who we will designate “mild tasters”) are in the middle of the range between tasters and non-tasters.
In fact, that’s exactly what they are.
Rather than having either the taster or non-taster receptors on the surface of their tongues, in mild tasters both types of receptors are expressed. That means that PTC is able to bind to roughly half of the available receptors on the tongues of these individuals, triggering a weakly bitter taste.
This is an example of incomplete dominance–a form of non-Mendelian inheritance.
Ordinarily with traits that follow Mendelian inheritance, the dominant allele completely masks the expression of the recessive allele. But in the case of the gene for this taste bud receptor, the dominant taster allele is not strong enough to completely shut out expression of the recessive non-taster allele.
It is not uncommon to find examples of incomplete dominance in plants. For example, when certain plants that breed true for red flowers are crossed with other plants of the same species that breed true for white flowers, the resulting offspring have pink flowers: a blending of the traits from each parent plant. This is seen in plant varieties including carnations, snapdragons, and four-o-clocks.
Using PTC Paper to Explore Genetics
The great thing about using PTC paper to study genetics is that unlike eye, hair, or skin color—you can’t look at a person and tell what trait they are going to have.
No one knows if they will be a taster, a non-taster, or a mild taster until they actually try to taste PTC.
You can order PTC paper online from vendors including Amazon or Home Science Tools. You can often find the paper sold in vials of 100 for only a few bucks! That means you can test the ability to taste PTC with a lot of people! (More on that later).
Once you have the PTC paper, testing is easy.
Experiment: Can You Taste PTC?
- Remove a single piece of PTC paper from the vial
- Holding one end of the paper, touch the other end to your tongue
- If you have a single copy of the taster allele (Tt), you should perceive a mildly bitter taste. If you have two copies of the taster allele (TT), the paper will taste extremely bitter
- If you taste nothing, turn the paper over and try touching the other side to your tongue. If you still taste nothing, you have two copies of the non-taster allele (tt)
Don’t stop there! Take this activity even further by tracing the ability to taste PTC through the members of your family. It’s a really fun way to see genetics and heredity in action.
For instance, there are four members in my immediate family: me, my husband, and our two sons. I can taste PTC and so can my younger son. My older son and my husband can’t taste PTC. Just knowing that information, I can discern the following:
Since my husband and older son can not taste PTC, I know that they both have two copies of the recessive non-taster allele. Otherwise, PTC paper would taste bitter to them.
Because my younger son and I can taste PTC, we must have at least one copy of the dominant taster allele. But how many copies of the allele do we have? Is there a way to tell? Yes, there is!
We can use Punnett Squares to “work backwards”.
A Punnett Square is a handy tool used to predict the outcome of a genetic cross if you know which alleles the parents have.
Since we don’t know for certain what alleles I have, I will create two Punnett Squares, one assuming I have two copies of the taster allele (TT) and the other assuming I have only one copy (Tt).
In each Punnett Square, my possible allele contributions are shown in purple, while the allele contributions my husband makes are shown in orange.
As you can see from the Punnett Square on the right, the only way for my husband and I to have produced a child unable to taste PTC is for me to have one copy of the taster allele and one copy of the non-taster allele (Tt). If I had two copies of the taster allele, there would be no way to produce a child that lacked the ability to taste PTC (as seen in the Punnett Square on the left). But because I have a copy of both gene alleles, I am able to pass either allele on to my children. Combined with my husband’s contribution of the non-taster allele, any children we produce would have an equal chance (50%) of being able or unable to taste PTC. And that’s exactly what happened in real life. Isn’t that fascinating?
Once you’re done experimenting
on with your immediate family, you can use all of the extra pieces of PTC paper to test your extended family members (grandparents, aunts, uncles, cousins, etc.). If you gather enough information, you may be able to construct a family pedigree.
A pedigree resembles a family tree, but allows you to track the inheritance of a given trait through multiple generations. For instance, here is a pedigree I made of my family to trace how the trait for left-handedness (a recessive trait) has been passed down through the two sides of my family.
Take it from an experienced science teacher: experimenting with PTC paper is always a hit with students. It’s so fun for me to watch as students take that hesitant taste and to watch their reactions.
If you’d like to learn more about DNA, you might like this post that covers DNA Basics: What’s So Special About DNA
To learn how our DNA controls the color of our hair (and why hair turns gray as we age), check out this post: Why Does Hair Turn Gray: The Science of Hair Color
If you have sun-sneezers in your life (folks who sneeze when exposed to bright light like the sun), you may be interested to know that this trait is also controlled by DNA. Learn more here: The Science of Sun Sneezing
To learn more about the Genetics of PTC and Bitter Taste, click here: PTC The Genetics of Bitter Taste
If you are looking for a resource to help you or your student learn about DNA, how it is expressed, and how DNA mutations can lead to diseases (including cancer), check out my self-paced, online course DNA Basics, Gene Expression, and DNA Mutations. In it, students will learn how DNA is organized inside cells and how it is expressed through cellular transcription and translation. Directions for hands-on labs and activities are included to enhance the learning process. Students will practice DNA base-pairing rules, transcription, and using the codon table to translate mRNA into protein.
In a related online course, Genetics and Heredity, students learn how DNA is expressed as observable traits. They learn about dominant and recessive gene alleles by exploring the traits they and their family members possess. Students learn the principles of Mendelian Genetics and how to use Punnett Squares to predict the outcome of monohybrid and dihybrid genetic crosses. Patterns of non-Mendelian inheritance, including incomplete dominance, codominance, sex-linked traits, polygenic inheritance, and the inheritance of mitochondrial DNA are also explained. Throughout the course, concepts are brought to life using real-world examples such as color-blindness, hemophilia, the genetics of human blood types, the genetics of calico cats, and much more.
** This is a simplified explanation of how eye color is manifested. The genetics of eye color is explored more completely in the self-paced, online course Genetics and Heredity.
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