Science for Kids: Population Genetics with PTC paper

I love going out and teaching kids about science.  Since moving from North Carolina to Indiana, I have really missed engaging in STEM outreach at local schools.  However a group of us at work decided we would be on the lookout for these opportunities and get into some schools.  Our company Inari has been supportive of our efforts and so we recently found a local middle school, East Tipp that welcomed us to take over an 8th grade science class for a day.

The group  at work has been amazing as we build out our activities and lessons.  The credit for this science module that delves into population genetics goes to Cole Davis.  He put together a great lesson!

Objective:  

The objective of this lesson is to learn more about about population genetics which is the study of gentic variation in populations.  Some of the concepts to tackle include gene frequency, dominant, recessive, homozygous, heterozygous, etc.  The core exercise involves tasting PTC (phenylthiocarbamide) paper and mapping out the class population genetics based on the whether each student can taste the bitter compound (PTC) or or not taste PTC.

Supplies:  

• Dice
• Calculator
• PTC paper strip
• Optional Worksheet:  Population Genetics Worksheet using PTC paper
  
  

The strips can be found very cheaply on Amazon: PTC Test strip link

Background:

DNA is how all of our genetic identity is stored and thus determines who are are and what we look like.   This DNA encodes for GENES and our genes are ultimately translated into many different  proteins.   Proteins define all the functions in our bodies or in a living organism. 

This figure shows how DNA is transcribed to RNA and RNA is translated into protein.

https://biologywise.com/protein-synthesis-process

The exercise today looks at one particular protein called a PTC taste receptor that is found in on the tongue.  We have lots of taste receptors that allow us to taste salty, sweet, bitter or sour.  Each different receptor is coded by our DNA.   One of those genes found coded in our DNA is for a PTC taste receptor that allows some of us to taste PTC.  If you can taste it tastes very bitter!  

https://learn.genetics.utah.edu/content/basics/ptc/

For around 75% of the population, PTC tastes very bitter!   For the other 25% of the population they do not taste any bitterness (or anything at all) when trying PTC.  

The PTC taste receptor protein found on our tongues looks like this:

Whether you taste PTC or not is determined by just one gene called TAS2R38.    The genes we get are determined by our parents.  For all genes, we all get one copy of a gene from our mom and one copy of a gene from dad.   Our parents DNA is ultimately what determines the makeup of our DNA and so our parents can give us different versions of the PTC gene depending on what they have.   The gene version that forms a protein that can taste PTC is a DOMINANT gene.    If we have just one gene from our parents that is dominant we can taste PTC.  However over time, the DNA code changed and that change means the DNA translated into a slightly different protein.   This changed version of the gene is RECESSIVE.   The new protein that some people have from a recessive version of the gene is not functional, meaning it does not work and the receptor does not taste the PTC.    However since you get a copy of the gene from mom and a copy from dad, you need to have both recessive copies (non-functional) in order to not taste PTC.

PTC gene:

A = Dominant (just one copy can taste PTC)

a = Recessive (takes two copies to NOT taste PTC)

Individuals have 2 copies of the genes:  One from each parent

AA = "Taster" since both copies are Dominant from each parent = Homozygous Dominant

Aa or aA = "Taster" since one parent still gave a dominant version of the gene - Heterozygous

aa = "Non Taster" since both parents gave a recessive version of the gene = Homozygous Recessive

EXERCISES

Exercise 1:   Taste the PTC paper

With the background out of the way, now the fun can begin.  The students can take the PTC paper and taste it.   At this point you see all kinds faces and get all kinds of reactions!

This just tastes like paper.

YUCK!

I don't taste anything

That was disgusting!

I didn't like that!

As long as you have a decent size population, you will have some tasters and non-tasters.   Statically 1 in every 4 people do NOT taste.  However, smaller populations can have very different frequencies.  Once everyone tastes you can dig into the genetics of the class.

Exercise 2:   Class Frequencies

The first thing to do is get the frequencies of the class population.  How many tasters in the class?  How many non-tasters?  After determining the class size you can determine the percentage of tasters (AA, Aa, or aA) vs the non-tasters (aa).  Did the class follow the average 75% taster / 25% non taster human population?  If not, why would the class frequency look different?

Exercise 3:   Hardy-Weinberg Equation

When we look at the class population data, we now know with certainty that the non-tasters are aa or homozygous recessive.  However everyone that tasted the PTC could either be AA (homozygous dominant) or Aa / aA heterozygous.   There is a mathematical formula that we can use to predict how many people in the class are AA vs heterozygous.  The formula is called the Hardy-Weinberg Equation and as a class we worked through solving it.  Following steps 1-5 below will solve this equation. 

Here is a resource where the equation is shown with a real example:  http://www.germanna.edu/wp-content/uploads/tutoring/handouts/Hardy-Weinberg-Equilibrium.pdf

Exercise 4:   Random Distribution vs Hardy-Weinberg Equation 

We just predicted the percentage of AA, heterozyous and aa frequencies in our class using the Hardy-Weinberg equation.   Now we tried to simulate natural variation in the population by rolling a dice so that each Taster could assign themselves as AA, Aa or aA.  The non tasters are still aa.

Each taster rolls a dice and based on there rolls they are the following.

1 or 2: aA

3 or 4: Aa

5 or 6: AA

Since each student now has a genetic identity for PTC based on the dice roll if they were tasters,  we re-calculated the frequencies and compared it to the Hardy-Weinberg result.  If there were differences we discussed the reason.  The results was usually much closer when we had a really big class.  In the smaller classes the two results were usually a little different. 

Exercise 5:  Bottlenecks

After working through the population genetics we talked some about bottle necking.   This is when a population is reduced to a smaller group or size and the limited diversity in the that population becomes the new norm.

 I work in agriculture and there has been intentional bottlenecking through artificial selection in some crops.   We talked a little through this concept.  This bottlenecking is one of the reason we have been able to increase yields.  

There are also examples where bottlenecking in agriculture has had consequences.  The potato famine is one example we talked through as a class.  The lack of genetic diversity led to a loss of the potato crop which in turn led to starvation and death.

From:  https://www.britannica.com/event/Great-Famine-Irish-history

The Irish relied on one or two types of potatoes, which meant that there wasn't much genetic variety in the plants (diversity is a factor that usually prevents an entire crop from being destroyed). In 1845 a strain of water mold accidentally arrived from North America and thrived in the unusually cool moist weather that year. It continued to destroy potato crops from 1846 to 1849. 

The PTC paper really helps to bring some fun into the concept of genetics.   There are some worksheets that Cole put together that allow the students to work through these exercises.   The worksheets contain even more details and scenarios depending on how deep you would like to go with this science lesson.

Worksheet:

Population Genetics Worksheet using PTC paper


Thanks to some awesome folks at Inari with huge props to Cole, Katie, Grant, Gretchen, Jess.  And thanks to East Tipp Middle School for inviting us!