The Tastebud Tango! (Carly Reifman)

Welcome to....

the Tastebud Tango!

Principle Investigated

Variation of taste-related traits distributed within a given population

Standard

HS-LS3-3. Apply concepts of statistics and probability to explain the variation and distribution

of expressed traits in a population

[Clarification Statement: Emphasis is on the use of mathematics to describe the probability of traits as it relates to genetic and environmental factors in the expression of traits.]

[Assessment Boundary: Assessment does not include Hardy-Weinberg calculations.]

Materials

Science students with a sense of taste

PTC (phenylthiocarbamide), sodium benzoate, and thiourea test paper strips (every student gets one of each)

Procedure

Each student gets a strip of paper without knowing what is on it. On a count of three, all students will place the strip completely on their tongues and close their mouths. For the first strip, students will record if they taste anything or not. For the second strip, students will again record if they taste anything or not, but this time if the student tastes something, he or she will record if that taste is either salty, sweet, or bitter. Finally, students will try the last strip and record if they taste anything or not. The class will use its collective data to come up with various taste statistics for each test strip.

Student prior knowledge

What is a gene? Students should understand the central dogma in genetics, which maps the flow of DNA to RNA to proteins. When a gene is copied to become a protein, we say that gene or allele is "expressed." We can measure or observe that expression and call it a "phenotype" or a "trait."

Students should also know how they received their own genes. That is, through meiosis in each parent that was passed on through sexual reproduction. Each person has two copies of every gene, as they received one from each parent.

Explanation

Because we receive copies of genetic material from each parent, we have two versions of every gene. Sometimes they are the same, but sometimes they are different. We learned about how genetic differences may come about in meiosis (like crossing over), but in this case, we will look at inheritability of specific traits.

Being able to taste phenylthiocarbamide (PTC) as bitter is a dominant trait. What is a dominant trait? It is when you only need one copy of that gene to show the trait. Let's designate the gene responsible for tasting PTC as bitter as "A" and the gene that lets you taste PTC as tasteless as "a." Remember, we get two copies of every gene, so here's what our genes would look like:

Note: homo is the Latin root for "the same" and hetero is the Latin root for "different."

So homozygous means alleles that are the same and heterozygous means alleles that are different.

The lowercase "a" is an example of a recessive gene. So, you can have "A" (dominant) and "a" (recessive) and still taste PTC as bitter because that is what a dominant gene does--it masks the recessive gene so you don't know you have it. You can get either gene from either parent.

Today we experimented with three types of chemicals absorbed onto paper test strips: PTC, sodium benzoate, and thiourea. The purpose of the experiment was to see firsthand the variation of traits in a given population (today's population was our class).

The first test strip we tried was thiourea. People will report to tasting it as extremely bitter, tolerably bitter, or tasteless. Researchers in a study at Yale University found that about 10-15% of their tested populations reported the taste of thiourea to be unbearably bitter and usually stayed away from bitter foods like dark-green leafy vegetables, coffee, or chocolate.

The second test strip we tried was sodium benzoate. People may report it as either tasteless or salty, sweet, or bitter. Two separate alleles are responsible for the tasting of this chemical compound, and you only need one copy of either to be able to taste it at all. If you cannot taste sodium benzoate, then you are homozygous recessive for the trait. A study found that 25% of its participants reported a distinct taste for a 0.1% solution of sodium benzoate.

The last strip we tasted was phenylthiocarbamide (PTC). About 70% of the world's population perceive PTC's taste as bitter and the other 30% as tasteless. Of course, this number is averaged across populations, because when we look at individual populations, the statistics change. For example, the number of Aboriginals (indigenous people of Australia) that taste PTC as bitter is as low as 58%, but that number is as high as 98% for indigenous people of the Americas. It is worth it to note that tasters of PTC may report it as either tolerably bitter or almost unbearably bitter depending on a person's tastebud sensitivity.

Interestingly, there is an interplay between the expression of the genes tasting for both PTC and sodium benzoate. They are inherited independently but may work together to create certain taste sensations. Of the 25% of the population listed about that could actually taste sodium benzoate, those that perceived PTC as bitter and sodium benzoate as salty reported interesting findings. Those individuals were more attracted than most to such tastes as sauerkraut, buttermilk, spinach, and turnips. However, those who perceived both sodium benzoate and PTC as bitter liked such aforementioned foods even less than average.

Questions & Answers

1. Sally performs the experiment in today's lab with her parents. When she gives the PTC paper to her parents, they twist their face in disgust and loudly proclaim that it tastes terrible. When Sally puts the PTC paper on her tongue, she doesn't taste a thing. Sally's father turns to Sally's mother and jokingly asks "so should I be sure Sally is my daughter?" Sally's father believes that because Sally doesn't share the trait with either of her parents that she must have inherited it from a different father. However, Sally knows from her biology class that being able to taste PTC as bitter is a dominant trait. What can Sally say to assure her father that he is indeed her father?

Answer: Both her parents could be heterozygous for the trait to sense the PTC paper as "bitter." Because being able to taste PTC as bitter is a dominant trait, a person only needs one copy of the dominant allele to express the phenotype. Each of Sally's parents must have passed on their recessive alleles to their daughter, making her unable to perceive PTC as bitter.

Bonus point if you mentioned that Sally's genotype is homozygous recessive!

2. By the same token, administration of PTC test paper strips used to be used for paternity tests before the advent of modern genetic testing. Let's say a family of three each tried a PTC test paper strip. Here's the data:

In that era, there would have been strong evidence that the father-in-question is not the true father of the boy. However, you are a biology student with biology student wisdom! Is it possible that the "father" is biologically related to this boy? Why?

Answer: Yes, it is possible that the "father" and the boy are related. Once again, being able to taste PTC as bitter is a dominant trait, so the boy could have received a dominant allele from his mother and the recessive allele from his father.

Bonus point if you mentioned that the boy's genotype is heterozygous!

3. If you went to a support group called "Bitter Food Haters Anonymous" and administered sodium benzoate and PTC test strips to each participant, what might happen? Is that population a representative sample of the rest of the world?

Answer: After some very angry group goers yell at you for handing out vile-tasting strips of paper to their fellow members, you would probably realize that all or most of the attendees have the trait that allows them to taste both sodium benzoate and PTC as bitter. If they are members of a support group for bitter food haters, then they are not a representative sample because these people are banding together due to shared genetic makeup.

Video

Here's an AP Biology class taste-testing PTC paper strips. Note the different reactions!!

Hilarious.

HEY!

If you want to get these "tasty" paper strips strips for yourself, click here!