Students will explore the use of Punnett squares and rudimentary statistical math through the lens of a real-life barley breeding scenario. Students will be introduced to the genetics of two barley varieties and will work towards an objective using Punnett squares and reasoning. In this activity, students will cross the Buck and Lightning varieties to create an F1, then cross two F1s to create an F2, to satisfy the objective. Finally, they will use a Chi-square calculator to evaluate a set of real-life data in comparison to their results. This lab assumes that students have already had a basic introduction to Punnett squares, their uses, and how to complete them. It is designed as a potential culmination of, and not a replacement for a genetics and Punnett square unit.
Barley is an important and ancient cereal crop grown worldwide. Barley breeders around the world work to develop barleys that are most useful for the varying purposes of farmers and consumers. A variety of breeding techniques are used to target desired traits in barley. While many aspects of barley genetics are more complex than can be understood with Punnett squares, there are other aspects that can be analyzed using Mendelian genetics, such as shown in this lesson. Whether a barley is 2-row or 6-row is controlled by a single gene, and the same goes for whether it is naked or covered. While the 2-row allele is dominant, the naked allele is recessive. Therefore, breeding for these two traits together poses an interesting challenge for students, as they will have to work through multiple generations of crossing to meet the objective.
Part 1: Buck x Lightning (F1) Cross
1. Provide each student with a copy of the Barley Punnett Squares Lab packet.
2. Discuss the background lab and the professions of breeding and genetics of the lab as a class.
3. Begin by discussing key vocabulary. Have students develop definitions and record them in their lab packet using prior knowledge and class notes as reference.
4. Watch the Crossing Buck and Lightning video as a class.
5. Instruct students to fill in the key information section of their lab packet while watching.
6. After watching, go over the bullet points as a class or have students check with a partner to ensure they got all the information and understand the scenario and objective.
7. Explain to students that the first step in breeding will be to cross the Buck and Lightning barley varieties.
8. Instruct students to complete the Punnett square crossing Buck and Lightning and answer the ratio questions listed below on their worksheet. Depending on the math level of your class, you may want to then walk students through the process of multiplying the Punnett square ratios together to predict the ratios of both traits together.
9. Review the answers to each of the Punnett Squares and ratio calculations as a class.
10. Introduce students to a 4×4 Punnett square if they haven’t seen one before. Provide students about 5 minutes to work the one on their worksheet and record the quantities for each allele combination.
11. After, discuss with the following questions as a class:
a. Why do the numbers from the big Punnett square match the numbers from multiplying the small ones together?
b. Did we meet the breeding objective with this cross? Why not?
c. What should we do next to get closer to the objective?
Part 2: The F2 Generation
1. Students will now use two F1 barleys to cross to continue trying to meet the objective. Provide students about 10 minutes to complete the next two small Punnett squares and phenotype quantities.
2. After, discuss with the following questions as a class:
a. Did we meet the breeding objective? How do we know?
b. Is it enough to make a decision based on phenotype alone?
3. Explain to students that a selfing plant (self-fertilization) does not cross-pollinate with other plants, it makes seed by pollinating itself (meaning the offspring has genetic material from the one parent). Barley is a selfing plant unless intentionally crossed by scientists or breeders. When breeding for a new barley with specific alleles, breeders use plants that are homozygous for the desired traits. If a heterozygous plant that is used for a certain allele, it would produce offspring (seed) with any possible combination of the dominant and recessive alleles. To ensure the seed maintains the same traits as the parent plant, homozygosity is needed. After discussing, give students 2 minutes to write this answer in their own words on their worksheet.
4. Explain to students that with barley being a selfing plant, we know that we want to use a heterozygous plant, next we will complete a 4×4 Punnett square to determine the proportion of plants produced from this cross with the desired allele.
5. Instruct students to complete the 4×4 Punnett square and answer the questions.
Part 3: Chi-Square Test
1. Explain to students that they will use a Chi-square test to analyze their results.
2. Discuss the following questions with students:
a. What is statistical significance mean, and why does it matter?
b. Why would we want to check our predicted results against what actually happened?
3. Instruct students to complete the chart on their worksheet to determine the expected percent of their data. Give students about 15 minutes to calculate their percentages and use this Chi-square calculator to check and note the viability of their results.
4. Debrief as a class about the process:
a. What did they think of it?
b. Was it difficult? Why or why not?
c. How could these statistical tools be used in other ways?
d. Would a breeding/genetics career be interesting to anyone? Why or why not?
5. Give students time to reflect and answer the final question.