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One form of genetic modification that has been used for centuries is artificial selection, or selective breeding. This is the selection of individuals or populations possessing desirable traits to produce the next generation. This process has given rise to many of the plants and animals we encounter every day, including: different breeds of dogs, some good at hunting and retrieving game and others that are specifically bred for companionship and living in a house; and different breeds of cattle, some bred for milk production and others bred specifically for beef. Corn has been developed and is constantly being improved to produce higher yields in drought conditions, resistance to pests, and other advantageous characteristics through this process. Artificial selection is also important in that it played an important role in the development of the theory of natural selection. An understanding of Mendelian inheritance and statistical analysis of the results of crosses is important in determining the genes of an organism and the ability to produce offspring with the desired traits.
In this lab, students will observe ears of corn from F2 plants produced by a dihybrid cross for the traits of kernel color and endosperm composition, which changes the shape of the kernel. The color blue is dominant to yellow and the starchy nonwrinkled endosperm is dominant to a sugary wrinkled endosperm. Students will hypothesize the genotypes of the parents (F1 generation). First looking at each trait independently, and then looking at the independent assortment or linkage between the two traits. These hypotheses will be used to make predictions that will be analyzed by collecting data and applying a chi-square test.
Science
Math
Students should be cautioned to be careful when handling ears of corn. If handled with care, ears may last many years, but may be damaged if handled roughly. Kernels may occasionally fall off of the ears, but may be glued back into place.
Length of Time for Preparation: 15 minutes Length of Time for Classroom Teaching: 2 hours
Preparation Procedure Lab Set-up
This lab is recommended to follow the teaching of Mendelian inheritance as students will need basic knowledge of inheritance and probability as well as the vocabulary associated with this subject and experimental procedures.
Students should be familiar with the following terms: allele, gene, hybrid, dihybrid, purebred, P generation, F1 generation, F2 generation, homozygous, heterozygous, dominant, recessive, independent assortment, and linked genes.
Students should also be able to predict ratios of offspring from monohybrid and dihybrid crosses using Punnett squares and/or the rules of probability.
A chi-square test is used to evaluate the proposed genotypes of the parent generation that produced the observable offspring. The proposed genotype represents the hypothesis. Probability is used to predict the number of individuals with different traits that would be expected in the next generation if the hypothesis is correct. The equation for performing a chi-square test is show in in Figure 1.
The chi-square value is compared with a critical value shown on the table in Figure 2.
Introduce the topic and assess students for prior understanding.
Let students discuss their ideas, and guide the discussion without telling them if they are right or wrong.
Guided Introduction Monohybrid 1. Instruct students to focus on the trait of kernel color. 2. Have them record which allele, either blue or yellow, that they infer is the dominant and recessive allele. 3. Instruct students to form a hypothesis of the parents’ (F1 generation) genotypes that would produce the ear they observed in the ears (F2 generation). 4. Have them predict the ratio of phenotypes that this cross would produce. 5. Students then need to determine the number out of 100 that should express each phenotype. 6. Students should then select 4 or 5 rows to count 100 kernels, recording the number of each phenotype. 7. Guide the class through a chi-square analysis of their data to determine if their prediction and hypothesis used to make it is statistically reasonable. 8. If the chi-square test suggests a new hypothesis, have the students come up with a new one, predict and run a chi-square test to see how it fits. 9. Have each group report their hypothesis, prediction and chi-square results.
Smooth vs. Round 10. Instruct students to repeat the above procedure focusing on the wrinkled and not wrinkled kernel trait. 11. If the chi-square test suggests a new hypothesis, have the students come up with a new one, predict and run a chi-square test to see how it fits. 12. Have each group report out their hypothesis, prediction and chi-square results.
Dihybrid Cross 13. Now, ask students to determine if they predict the traits are inherited separately or if they might be linked or influencing each other in some way. 14. As a class, determine how the degrees of freedom will be different in this chi-square test. With four possible phenotypes, blue smooth, blue shriveled, yellow smooth, and yellow shriveled, there are now 3 degrees of freedom. 15. If the chi-square test suggests a new hypothesis, have the students come up with a new one, predict and run a chi-square test to see how it fits. 16. Have each group report their hypothesis, prediction and chi-square results.
A lab group received an F2 ear of corn that appeared to have equal numbers of blue and yellow kernels.
To learn more about agriculture careers, visit https://agexplorer.ffa.org/.
Any educator electing to perform demonstrations is expected to follow NSTA Minimum Safety Practices and Regulations for Demonstrations, Experiments, and Workshops, which are available at https://static.nsta.org/pdfs/MinimumSafetyPracticesAndRegulations.pdf, as well as all school policies and rules and all state and federal laws, regulations, codes and professional standards. Educators are responsible for abiding appropriate legal standards and better professional practices under a duty of care to make laboratories and demonstrations in and out of the classroom as safe as possible. If in doubt, do not perform the demonstrations.
This lesson is the work product of the Kansas Corn Commission. Our lessons are written in collaboration with Kansas teachers for use in the classroom. Teachers may copy and share this curriculum. Use of this product for commercial or promotional use is prohibited without express permission of Kansas Corn.
As the need to produce more food with less resources grows, it is important that future generations have a better understanding of agriculture and how corn farming and agriculture fit into our daily lives. That’s why the Kansas Corn STEM program provides lessons to teach science through the lens of corn. We are committed to providing free materials and training to support educators.
