Carolina LabSheets™
Overview
In this lab, students test corn pollen for evidence of the segregation of alleles. The gene locus is waxy endosperm on chromosome 9. Students should be familiar with the iodine test for starch before performing this lab.
Needed Materials
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Starchy: Waxy Pollen (177010) for up to 25 students
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Iodine Solution (177020)*
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watch glasses (742300) or petri dishes
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dropping pipets
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microscope slides
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coverslips
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Bibulous Paper (634050) or paper towel
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microscopes
*You may substitute Iodine–Potassium Iodide (869051) or Lugol Solution (872793) for the 177020 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 allergic to corn or corn pollen.
Procedures
Have students work individually or in pairs.
With the vial of pollen securely capped, agitate the vial to suspend the pollen. Uncap the vial and pour suspended pollen into a watch glass or petri dish. Prepare a dish for each pollen workstation. The pollen settles fairly quickly. You may have to resuspend the pollen for each dish.
Set up workstations with the following:
| For pickup of corn pollen | For staining |
| dish of Starchy:Waxy Pollen | iodine solution |
| dropping pipet | coverslips |
| microscope slides |
Optional: Have students count and record the two colors of pollen grains, compile a class total, and calculate a chi-square value to test the 1:1 ratio. Students might also construct a Punnet square showing the expected F2 of crossing wx+/wx with itself. This activity may be done by itself or in conjunction with our Carolina LabSheets™: Testing to Reveal a Phenotype.
Answer Key to Questions Asked on the Student Labsheet
Expected results if alleles wx+ and wx are segregating.
About half of the pollen grains will stain dark blue with iodine, and the other half will stain reddish-brown.
Expected results if alleles wx+ and wx are not segregating.
All pollen grains will be the same color after staining with iodine.
Do your observations indicate that alleles wx+ and wx are segregating, or not? Explain.
Yes, alleles wx+ and wx are segregating. We see approximately equal numbers of dark blue and reddish-brown pollen grains after staining with iodine.
What was the purpose of the unstained slide?
To show that iodine did change the color of the pollen. Without this slide, it is possible to argue that the plant was producing pollen of two different colors and that the colors were unrelated to the type of starch produced. It serves as a negative control.
How would examining pollen from homozygous wx+/ wx+ and homozygous wx/wx plants contribute to this investigation?
It would tend to confirm that the pollen colors after staining result from the presence of different alleles. It would provide positive controls.
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2 Comments
What materials are required for conducting the experiment, and why is familiarity with the iodine test for starch important?
The materials needed to conduct this experiment are listed and linked, to the Carolina website, to this particular article.