Science Quest: How to Determine the Frequency of a Trait

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The topic of genetics is a mainstay of any life science or biology course. Sometimes it’s hard to find just the right activity or lab to reinforce the concepts especially those suited for small groups like homeschooling families. For that reason, I have plans to share science activities and labs to make them more accessible for homeschoolers. Today’s Science Quest: How to Determine the Frequency of a Trait.

Mendelian Genetics

Gregor Mendel was a monk who worked in the mid-1800s on the problem of how parents handed down traits to their offspring. He bred pea plants in deliberate ways and observed many generations of peas to make conclusions about how pea traits are passed down from their parent plants. Mendel is responsible for developing three laws:

• Law of Segregation– Given two hereditary factors (alleles) for each trait which must segregate during gamete production so that offspring get only one factor from each parent.
• Law of Independent Assortment– Genes for different traits can segregate independently during gamete (sex cell) formation.
• Law of Dominance– Some alleles are dominant while others are recessive; an organism with at least one dominant allele will display the effect of the dominant allele.

In this lab activity, we’re going to observe some traits that are Mendelian in nature. That is, they don’t behave differently than a simple dominant/recessive trait.

Vocabulary to Know to Study the Frequency of Traits

This activity should be done after you have studied some basics of genetics and are already familiar with the following terms:

• gene
• allele
• genotype
• phenotype
• homozygous
• heterozygous
• frequency
• dominant
• recessive
• population

Once your student has a handle on how traits are passed down to new generations and they know about how a Punnett Square works, they can talk about the frequency with which traits appear in offspring.

Human Genetic Traits to Survey

To do this investigation, there are plenty of human traits to choose from. They must be simple Mendelian traits which have two phenotypes. Multiple allele traits like eye color and sex linked traits like color blindness follow different rules. Here’s a list of options:

• Tongue Roll– Whether or not you can roll your tongue into a U shape
• Eyelash Length– They come in short or long and in case you don’t know which kind you have, consider whether or not you’ve every gotten your eyelashes tangled.
• Earlobes– Attached or unattached. Is there space between the end of your ear and the side of your head?
• Hand Clasp– Put your hands together and see whether your left or right thumb is on top.
• Pinky Shape– If you turn your hands palms out and put your pinkies together, are they side by side straight? Or do they bend away from each other at the top?
• Dimples– You either have them or you don’t.
• Cleft Chin– Is there a dimple in your chin or not?
• Widows Peak– A section of hair which grows in a point on your forehead
• Thumb Bending– Hitchhiker’s thumbs bend backwards while others will only go straight
• Finger Hair– Does it grow between the knuckle and the end of your finger or no?
• Toe Length– Is your second toe longer than your first or vice versa?
• Vulcan Greeting– Can you do the Vulcan greeting? Yes, this is a genetic trait!
• Handedness– Right or left handed?
• Freckles– Either you have them on your face or you don’t. (I do!)

Collecting Data

There’s no right or wrong way to organize your data for this survey. You do want students to be able to work with their data later on and it’s helpful to discuss the information they need. Remember that printables aren’t necessary! Just grab a piece of notebook paper and get to work. Here are some other things to note:

• Determine which traits you will survey– This will tell you something about what you’ll be recording in your chart.
• Discuss your sample size– As with any scientific investigation, your sample size cannot be too small. What size would be appropriate for this survey?
• Prepare your data chart– You can use rulers to keep things neat, but it’s not necessary as long as you can interpret the data later on.
• Who and how will you survey– With the internet at our disposal, it’s easier than ever to get lots of people to help you out.
• Collect just the answer to which trait– gender is not necessary because I’ve asked you not to choose sex-linked traits. You could ask for gender and see if whether it makes a difference in the frequency of the trait particularly if you also choose a sex-linked trait. Then your students can determine whether they think it is sex-linked based on the frequency between male and female populations.
• Collect the data– And record it in your chart

Teaching the Hardy-Weinberg Principle

Mendel observed that both traits if they were present in the parents would show up in the offspring of every generation. Under normal circumstances, will the frequency of these traits change from generation to generation? The Hardy-Weinberg Princple says no. Allele or genotype frequencies in a population will stay constant from generation to generation (in the absence of other evolutionary influences). In 1908, G.H. Hardy and Wilhelm Weinberg were able to prove that the alleles of both the heterozygous and the homozygous genotypes show up in generations with the same frequency.

There is a mathematical formula for demonstrating the frequency of genotypes in a population and once you’ve collected the data on your traits, you can use it to test the frequency of the genotypes in the population you sample. I’ve included the equation below.

p² + 2pq + q² = 1

p =  frequency of the dominant allele in the population
q =  frequency of the recessive allele in the population
p2 = frequency of homozygous dominant allele in the population
q2 = frequency of the homozygous recessive allele in the population
2pq = frequency of heterozygous allele in the population
p + q = 1 represents all the allele for a trait in a population.

The purpose of the Hardy-Weinberg equation is to tell us the percentage of our sample population that is homozygous and heterozygous for the trait. Below is a Khan Academy video on The Hardy-Weinberg Principle. It will demonstrate how to use the equation for your surveyed traits. This is a great exercise for those in high school math.

 

We totaled the data for each trait and worked through the equation to determine the percentage of the frequency of a particular allele in our sample population.

Some questions you can ask your student for any trait on which you’ve collected data:

• What is the size of your sample population?
• Determine the percentage of your population that exhibit each trait
• Name the genotypes of each trait.
• Use the Hardy-Weinberg formula to determine what percent of the population are homozygous for a trait.
• Use the formula again to calculate the percent of the population which are heterozygous for a trait.
• Conclude whether or not you think the percentages would change for other traits or different sample populations.

Resources for Teaching Mendelian Genetics

We use a variety of resources for biology in our homeschool. Since my professional background is biology and education, there is an unwritten rule that we do not buy biology curriculum. To be honest, it’s true for any of the sciences. It makes it more difficult to blog about our science instruction, but I’m committed to sharing more about it because I think it’s helpful to see that science isn’t scary and it doesn’t require special curriculum. Here are a few resources I used for this unit:

• CK-12 High School Biology– Our high school biology text. There is a lot of material here and there are videos embedded within the text along with practice, quizzes, and activities on the website.
• CK-12 Middle School Life Science– I like this book a lot. I have a small collection of middle school life science books and none of them are challenging enough. This one is great. It’s not high school level, but it doesn’t leave things out.
• Gregor Mendel: The Friar Who Grew Peas– Biographical picture book about Gregor Mendel which explains his theories and how he developed them. It also gives a great introduction to the frequency of a trait over multiple generations.
• Genetics by Amy Brown– This is a great little unit on genetics written by a teacher. It comes with everything you need and I especially liked the Power Point presentation. It was a nice go along with our CK-12 texts.
• Activities for The Friar Who Grew Peas– If you want a simpler approach, you can focus on dominant and recessive traits only and even grow your own peas. Stephanie also has some fun notebooking pages to go with this book.