Middle School Biology: Genetics

This series of lessons for middle school students is strongly influenced by the excellent STC/MS curriculum Organisms – from Macro to Micro. Having taught this curriculum on multiple occasions, I’ve made some significant modifications and extensions. For instance, I put greater emphasis on the very large number of seeds produced by plants and the ways that they are dispersed, as this is a good opportunity to discuss important evolutionary and ecological concepts. Perhaps more importantly, the Fast Plants used to investigate genetic ratios show incomplete dominance (with a 1:2:1 ratio). In the curriculum, it suggests organizing student observations in such a way that they get the traditional 3:1 ratio of a trait with complete dominance. I find that students actually respond very well to introducing incomplete dominance first, since incomplete dominance connects very directly to what the alleles are actually doing and then introducing complete dominance second. In this case, I’ve used genetic corn to introduce complete dominance in a hands-on, but convenient, way.

These lessons come relatively late in the year. Students have already learned about mitosis and have some knowledge of DNA structure and function.

I teach in unusually long class periods, so other teaches would likely need to split the lessons up further.

Student handouts are included at the end.

Introduction to Genetics:
Setting the Stage

Getting Started
• Do Now: Estimate how many seeds are in an average bell pepper. What would happen if all of these seeds sprouted in the same location?
• Check homework: randomly select students to summarize the main ideas in “Growing Seedless Fruits.”
• Randomly divide class into lab groups.

Seed Number and Dispersal Techniques
• Give each lab group seeds from one species. Within the class there should be wind, water, and animal dispersed seeds. Ask groups to write down one or more hypothesis to explain how these seeds are spread in nature. How could they test their hypothesis?
• Class discussion and notes: work together to write notes about how seeds are spread. Discuss ideas for testing hypotheses, but don’t actually carry out the experiments (unless they are exceptionally quick).
• How many seeds does a plant make? Show bell pepper. Share estimates made in Do Now. Give number of seeds actually in the pepper (count seeds before class). How many offspring do you think will live to adulthood, on average? What implications does this have?
• Show “Exploding Cucumber” video from Earth Unplugged. https://youtu.be/wOIHzl2h9a8
• Design your own fictional fruit – how will your fruit spread seeds effectively? Draw a picture and write a brief description. Present to class.

Harvest Fast Plant Seeds
• Review Fast Plants – how many seeds were planted in each pot, how many seeds germinated, and what was done to care for the plants.
• Demonstrate how to collect seeds from Fast Plants.
• Distribute Fast Plants and allow time for each group to collect and count seeds.
• Share seed numbers and any other observations with class.

Extension (Use if Class Finishes Early)
• Many seeds only germinate well after they have passed through an animal. Design an experiment with your lab partner to find out what part of the animal’s digestive tract is most important for stimulating germination. Share. (These experiments will not be carried out, so they may require techniques that are not practical in the classroom. They should be well reasoned and logical. If an experiment involves animal cruelty discuss why it would be important to find a different method even if that experiment would be informative.)

Homework
• How would your life be different if you had as many siblings as a Fast Plant? Write at least 150 words and be prepared to share your response with the class.

Materials
Carolina Biological seed set, bell pepper (count seeds before class), screen and internet connection to show “Exploding Cucumber” by Earth Unplugged, Organisms – Macro to Micro Student Guide and Source Book (for homework), mature Fast Plants (planted and pollinated in earlier lessons), containers for Fast Plant seeds

Introduction to Genetics:
Discovering a Pattern

Getting Started
• Do Now: Go to the notes you made about Fast Plants. What color were the leaves and stems of the newly-germinated seedlings?
• Check homework: randomly select students to share their responses to the prompt “How would your life be different if you had as many siblings as a Fast Plant?” Briefly discuss.
• Randomly divide class into lab groups.

Collect and Discuss Fast Plant Color Data
• Explain that Fast Plant seeds collected in prior class have been planted and exposed to bright light over the weekend.
• Give each lab group two petri dishes of Fast Plant seedlings. (Each petri dish contains about 50 seedlings.) Ask them to come up with ways to categorize the seedlings based on color.
• Share the categories students come up with. Discuss if necessary to arrive at categories similar to “All Green”, “All Purple”, and “Part Green/Part Purple”.
• Lab groups score their seedlings for color and share data in class chart.
• Give lab groups time to work together on the following questions:
o What ratio can you use do describe the different colors of seedling? (Use the data for the entire class.)
o Can you simplify this ratio? If you round the numbers to the nearest 10 or nearest 100 does that help you simplify the ratio?
o Is the ratio for your lab group the same as the ratio for the entire class? Which ratio seems more important? Why?
• Discuss the questions as a class. Guide students to:
o Discover the 1:2:1 ratio (all green : part green/part purple : all purple) present in the Fast Plant seedlings.
o Understand that a larger sample size is generally going to produce more reliable data.
• Compare the seedlings we counted today (offspring) with the appearance of the original seedlings (parents). Discuss any hypotheses that students may have to explain this situation.
Read “Mendel’s Discoveries” (in Organisms – from Macro to Micro)

• Do a shared reading with “Mendel’s Discoveries.”
• Conduct a class discussion to draw connections between what Mendel discovered and what we observed with the Fast Plant seedlings. If students don’t notice difference between Mendel’s ratio (3:1) and the class ratio (1:2:1), highlight this.

Read “Heredity – Passing it On” (in Organisms – from Macro to Micro)

• Do a shared reading with “Heredity – Passing it On.”
• Conduct a class discussion in which students use the information from both readings to explain the ratio they observed with the Fast Plant seedlings.
• As a class, write notes. Be sure to cover the following information:
o Fast Plants seem to have a trait for color on each chromosome. If both traits are green, plant is all green. If both traits are purple, plant is all purple. If one trait is green and one is purple, plant is part green and part purple.
o When Mendel observed height in pea plants, if both traits were tall, the plants were tall. If both traits were short, the plants were short. If one trait was tall and one trait was short, the plants were tall.
o Define heterozygous, homozygous, dominant, recessive, phenotype, and genotype.

Homework
• Using the words heterozygous, homozygous, dominant, and recessive summarize what Mendel observed when he looked at seed color in pea plants.

Materials
Fast plant seedlings, Organisms – Macro to Micro Student Guide and Source Book

Introduction to Genetics:
Does the Pattern Continue?

Getting Started
• Do Now: Pick up a copy of the pop quiz by the door. You have 4 minutes to complete it. (Copy of the pop quiz is included at the bottom of this lesson plan.)
• Randomly divide class into lab groups.

Collect and Discuss Genetic Corn Data
• Explain that in a corn cob, each kernel is a seed produced by a separate fertilization. Introduce genetic corn cobs (3:1 ratio; purple : yellow kernels). Give hints about how to effectively score kernels for color.
• Allow class to decide how many kernels each group should score for color (only accept answers of 50 or more kernels).
• Each student writes hypothesis for what ratio the class will find, in consultation with lab partner.
• Give time for each lab group to collect corn kernel color data and add it to a class data chart.
• Discuss corn kernel data and collaborate as a class to write notes.
o Approximately what purple : yellow color ratio did the class find?
o Was the ratio for the class the same as the ratio found by each lab group? What does this suggest?
o Is the pattern of inheritance in corn more like what Mendel observed in pea plants or more like what we observed with Fast Plants?
o Define incomplete dominance and complete dominance.

Read “What Are the Chances?”
• Do a shared reading of “What Are the Chances?”
• Conduct a class discussion about how to use Punnett squares.
• Give simple inheritance questions for students to solve using Punnett squares.
o A homozygous green Fast Plant and a homozygous purple Fast Plant are crossed. What color are the offspring likely to be?
o Two heterozygous tall pea plants are crossed. What are the likely heights of the offspring?
o A Fast Plant that is heterozygous for color is crossed with a Fast Plant that is homozygous green. What are the likely colors of the offspring?
o A heterozygous purple-flowered pea plant is crossed with a white-flowered pea plant. What are the likely flower colors of the offspring?
o Define genotype and phenotype.

Punnett Square Worksheet
• Give students worksheet to practice Punnett Square problems. (Worksheet below.)

Homework
• Complete any remaining Punnett Square problems on the worksheet that you were unable to finish in class. Worksheet will be collected and graded.

Materials
Pop quiz, genetic corn cobs (showing 3:1 ratios), Organisms – Macro to Micro Student Guide and Source Book, Punnett square worksheet

Bibliography
Milne, Henry. Organisms – from Macro to Micro: Teacher’s Guide. Ed. Lorraine Coffey. Burlington, NC: Carolina Biological Supply, 2006. Print.

Organisms– from Macro to Micro: Student Guide and Source Book. Burlington, NC: Carolina Biological Supply, 2003. Print.

Name ______________________

Pop Quiz

Instructions:
Write the words from the word bank next to the best definition.

Word Bank:
Heterozygous, Homozygous, Dominant, Recessive

1. A trait that is visible when an organism has either one or two copies of it.

________________________

2. An organism that contains two different versions of a trait.

________________________
3. A trait that is visible only when an organism has two copies of it.

________________________
4. An organism that contains two copies of the same version of a trait.

________________________

Name ____________________

Punnett Square Practice Problems

1. There are two types of earwax, “wet” and “dry.” Wet earwax is dominant to dry earwax. (Dry earwax is found primarily in Asians and Native Americans. Almost everyone else has wet earwax.) If a mother who is heterozygous dominant and a father who shows the recessive trait have children, what is the probability that their first child will have dry earwax?

2. Labrador Retrievers come in several colors including black and chocolate. In these dogs, black color is dominant to chocolate color. If a black Lab that is heterozygous for coat color is crossed with a chocolate Lab, what colors could the puppies be? Theoretically, what percentage of puppies would have each coat color?

3. In rabbits, agouti fur is dominant to black fur. (The term “agouti” means that each individual hair has bands of different colors.) Two agouti rabbits are crossed and produce 6 offspring. Four of the baby bunnies are agouti and 2 are black. How is this possible?

4. One form of polydactyly in humans is dominant. (Polydactyly refers to having more than 5 fingers or toes.) If a father who is homozygous for polydactyly has two children with a mother who does not have polydactly, how many of the children would we expect to have extra digits?
5. In fruit flies, apterous individuals lack wings while wild type individuals possess wings. Wild type is dominant to apterous. If a wild type fly is crossed with an apterous fly and about half of the offspring are apterous while the rest of the offspring are wild type, what would the most likely genotypes of the parents be?

6. In Fast Plants, the rosette form (which is especially short) is recessive to the standard form. Describe an experiment to determine if a rosette plant is homozygous or heterozygous for this trait.

7. In cyclopes, the eye may be purple or orange. Orange is dominant. An orange-eyed cyclops would really like to have offspring with purple eyes. Is this possible? Give this cyclops advice about how to maximize her chances of having a purple-eyed cyclops baby.

8. In merfolk, fancy tails with ruffled edges are dominant to plain tails with smooth edges. A plain-tailed mermaid is going to have a baby with a fancy-tailed merman. They are very curious about what sort of tail their baby is likely to have. Can you tell them what to expect?

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