Overview
A phenotype is an expressed trait. Most phenotypes studied in introductory genetics classes are visually conspicuous traits such as wing shape or eye color. In this activity, students investigate a phenotype that is not so conspicuous, and they perform a simple chemical test to reveal the underlying basis of the phenotype.
Corn plants normally produce both amyloid and amylopectin starches, which accumulate in the endosperm of the seeds. A recessive mutation called “waxy,” (location 59 on chromosome 9), when homozygous, blocks amyloid starch production such that only amylopectin starch accumulates in the endosperm. This corn is called the waxy phenotype because the seeds have a dull, waxy appearance, compared with normal starchy seeds, which are more translucent. Testing with iodine clearly distinguishes between the two. Amyloid starch reacts with iodine to give a blue-black color, and amylopectin (waxy) starch reacts to give a reddish-brown color. In practice, the endosperm of wild-type starchy seeds will stain almost black, while the endosperm of waxy seeds will be much paler.
The F2 ears used in this activity show a 3:1 ratio of starchy seeds to waxy seeds and can be analyzed as a monohybrid cross, although that is not the purpose of this lab activity. The genotypes of the seeds are wx+/wx+ (homozygous starchy endosperm) wx+/wx (heterozygous starchy endosperm), and wx/wx (homozygous waxy endosperm).
This activity assumes that students are familiar with the iodine test for starch. If they are not, consider either testing a beaker of starch solution with iodine as a class demonstration, or have students test starch solution with iodine as a pre-lab activity.
Needed Materials*
- Wx Endosperm Alleles 3:1 Corn Ears (176550), 1 per team
- Iodine Solution (177020)
- dropping pipets
- sandpaper, medium to coarse grit
- paper towel (preferably, slightly dampened)
Optional Materials
Newsprint to cover and protect the work surface during sanding. Beakers or cups for the iodine solution. (Iodine penetrates many plastics, so store iodine solution in glass only.) Trays help contain any iodine stain that drips from the seeds. Iodine–Potassium Iodide (869051) or Lugol Solution (872793) may be used instead of the iodine solution.
Safety
Ensure that students understand and adhere to safe laboratory practices when performing any activity in the classroom or lab. Demonstrate the protocol for correctly using the instruments and materials necessary to complete the activities, and emphasize the importance of proper usage. Use personal protective equipment such as safety glasses or goggles, gloves, and aprons when appropriate. Model proper laboratory safety practices for your students and require them to adhere to all laboratory safety rules. Avoid contact of iodine solution with the eyes, face, nose, or mouth. Iodine solution will stain skin and clothes.
Ensure that none of your students is sensitive or allergic to grain dust or to corn in general. Students with sensitive skin or allergies may need to wear laboratory gloves and dust masks.
Procedures
Students may work individually or in pairs.
Remove the wrappers from the ears of corn. Cut a 2” square of sandpaper for each group.
Each student group needs the following:
- corn ear
- iodine solution
- pipet
- sandpaper paper
- towel (preferably damp, for removing sanding dust from kernels)
Optional: This LabSheet activity can be done by itself or in conjunction with our Testing for Segregation of Alleles LabSheet. As mentioned in the introduction, the F2 ears used in this activity can be analyzed as a monohybrid cross. Have students research and report on the biochemical pathways involved in starch production. They might also research the types of starches produced by other grains such as rice, sorghum, and wheat and by starchy, nongrain crops such as potatoes and cassava. How do these starches react when tested with iodine?
Answer Key to Questions Asked on the Student Labsheet
Examine the seeds in three or four rows on the ear. Note here any consistent differences in the appearance (phenotypes) of the seeds.
Most of the seeds are shiny and golden yellow, but some are a more dull, opaque yellow. Students will describe the seeds in various ways, but they should be able to distinguish two phenotypes.
What color does iodine produce when it reacts with starch? The starch turns dark blue-black or black.
What does the difference in the color developed by the iodine tell you about the endosperm of the different seeds?
It indicates that there is a difference in the chemical composition of the endosperm of the two types of seeds. Note: Be aware that if the students do not clean the dust from the ears, particles of starchy endosperm may adhere to the surfaces of the waxy endosperm and obscure the results.
If students are familiar with the iodine test for starch, they may answer along this line: “The endosperm of the darker-staining seeds contains starch, and the endosperm of the seeds that do not stain dark does not contain starch.” You may elaborate on the distinction between amyloid and amylopectin starch as you see fit.
Look at the phenotypes of the seeds. Do these show a correlation with the color developed after treatment with iodine? What might this indicate?
There is a correlation. Seeds that are golden-yellow stained dark, while the seeds that are dull yellow did not stain dark. This suggests that the phenotypes result from differences in chemical composition of the endosperms of the seeds.
Assuming that the phenotypes result from the action of a pair of alleles at a single gene locus, give the genotypes and their resulting phenotypes.
homozygous dominant shiny golden-yellow seed
heterozygous dominant shiny golden-yellow seed
homozygous recessive dull yellow seed
Describe the two phenotypes you have worked with in this lab. How did the iodine test help indicate the phenotypes?
Most kernels were bright and shiny. Others were dull and waxy. The phenotypes were easier to distinguish after the iodine test stained the two types of endosperm conspicuously differently.
Note: The students will probably first describe the outward appearance and then the test results. Remember that one goal of this activity is for students to learn that phenotypes include many traits besides those affecting outward appearance.
