Traits in Corn Cobs

Explore what students will learn and check out the additional resources that go along with the labs.

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Overview

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. An example of this is breeds of dog, where some are good at hunting and retrieving and some are bred for companionship within a home. Another example are breeds of cattle, where some are bred for milk production and others, for beef. A third example is corn, a crop that has been developed over the years and is currently being improved to produce higher yields in drought conditions, resistance to pests, and other advantageous characteristics through this process. Artificial selection also 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 two traits (1) kernel color and (2)endosperm composition. For kernel color, purple is dominant to yellow. For endosperm composition which affects kernel shape, the starchy nonwrinkled endosperm is dominant to a sugary wrinkled endosperm.
Hypothesize the genotypes of the parents (F1 generation).

Science

MS-LS3-1. Develop and use a model to describe why structural changes to genes (mutations) located on chromosomes may affect proteins and may result in harmful, beneficial, or neutral effects to the structure and function of the organism.
MS-LS3-2. Develop and use a model to describe why asexual reproduction results in offspring with identical genetic information and sexual reproduction results in offspring with genetic variation.

Learning Objectives

Students will observe phenotypic traits in corn kernels.
Students will count and record data to determine trait ratios.
Students will use Punnett squares to predict possible genotypes.
Students will understand dominant and recessive traits using a real-world example.

Materials (per group of 2-4 students)

Traits of Corn Cobs PowerPoint
1- Monohybrid corn ears from Carolina Biological, R Color (item #176500)
1- Su Endosperm Alleles 3:1 (item #176540)
Hand lens or magnifying glass (optional)
Copies of Corn Kernel Genetics Worksheet (one for each student) (pages S1-S2)
Calculator (optional)
Pen or pencil

Safety Considerations

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.

Procedures for Instruction

Length of Time for Preparation: 15 minutes
Length of Time for Classroom Teaching: 1 class period (45–60 minutes)

Prior Knowledge

It is recommended to follow the teaching of Mendelian inheritance for this lab as students will need basic knowledge of inheritance and probability as well as the vocabulary associated with this subject and experimental procedures.

Lab Set-up

Group or pair students so that there are enough materials for each group.
Each lab group will need one dihybrid ear. Do not remove the plastic wrap.

Classroom Discussion

Notes for the teacher on how to introduce the topic and how to assess students for prior understanding.

Have students observe the ears of corn and record observations.
Have each group report one observation to be recorded collaboratively as a class.
Students should report that there are yellow and purple kernels.
They should also observe more purple than yellow kernels.

Questions to pose to students after they’ve observed the cobs, without telling them if they are right or wrong.

What characteristics do you see that are different from what you are used to seeing?
How many different types of kernels do you see?
Which traits seem to be more common? Less common?
Which allele for each trait do you think would be dominant or recessive? Why do you think so?
If your answer is correct, what ratios for each phenotype should be observed?
Do the traits appear to be linked or inherited independently?

Background

Corn kernels can have different colors (e.g., purple or yellow) and textures (smooth or wrinkled). These traits follow Mendelian inheritance, meaning they can be dominant or recessive. Each kernel on a corn cob is a separate offspring resulting from pollination, so a single cob gives a large sample for data collection.

Purple is dominant to yellow.
Smooth is dominant to wrinkled.

Procedure for Lab

Pass out Corn Kernel Genetics Worksheet (students will be recording observations and answers here through-out the lab)
Observe the Corn Cob
- Examine the kernels and identify the two traits you’ll observe.
- Kernel Color: purple or yellow
- Kernel Texture: smooth or wrinkled
Count and Record on your Corn Kernel Genetics Worksheet
- Count at least 100 kernels (or the whole cob if it’s short).
- Tally how many kernels show each trait.
  - Example:
    75 purple, 25 yellow
    80 smooth, 20 wrinkled
Calculate Ratios
- Find the ratio of dominant to recessive traits.
- Example: 75 purple : 25 yellow = 3:1 ratio
Interpret the Results
- Use the ratios to determine the likely genotypes of the parent plants.
- If you see a 3:1 ratio, this suggests a cross between two heterozygous (Rr x Rr) parents.
Punnett Square Practice
- Complete a Punnett square for one of the traits below.
- Use symbols like:
  - R = dominant allele (purple)
  - r = recessive allele (yellow)

Discuss what fraction of offspring show the dominant trait.

Analysis Questions

What trait was dominant? How do you know?
What was the ratio of dominant to recessive traits?
What genotype combinations could have produced your results?
How does this relate to Mendel’s laws?
If you grew corn from only the yellow kernels, what would you expect in the next generation?

Extension

Compare two different cobs with different ratios.
Graph your data as a pie chart or bar graph.
Discuss environmental vs. genetic traits in plants.

Assessment

Hand out the student assessment sheet pages S1-S2. This assessment tool can be used to learn if learning out-comes were achieved.

Disclaimer

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.