Materials Included In Kit

Safety Precautions

Caution students about use of marker pins. The kernels are hard and pins cannot be easily inserted into the kernels. Pins should be inserted into the soft corn cob tissue of each ear.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All materials from this activity can be reused.

Teacher Tips

• All materials are reusable.

• The genetic corn in this kit can be used for years if handled carefully. Discourage the removal of kernels and have students place their pins carefully at the ends of the ears and not into the actual kernels.
• Be sure you have covered the basics of Mendelian Genetics before using this F₂ genetic corn. Genotype, phenotype, monohybrid crosses, and predicted ratios should be completed before exploring the dihybrid cross principles illustrated by the genetic corn in this kit.
• The purple to non-purple (3:1) and the smooth to wrinkled (3:1) alleles act independently of each other due to the independent assortment of the alleles during meiosis and the production of gametes. This fact is imbedded in the expected F₂ ratios and can be discovered by students.
• This exercise is easily extended to include the use of the Chi-Square test to determine the significance of any differences between expected and actual results. The expected results can easily be calculated after actual counts are made. The use of Chi-Square will be dependent upon your course goals and your student population.
• You can code your ears of corn and compare student counts from class to class and year to year as a check on the accuracy of student work.

Sample Data

I. Predictions

II. Actual Count

Answers to Questions

1. Which cross most likely produced your ear of corn? Include evidence to support your answer.
   

   Answers will vary. The actual phenotypic ratio should more closely resemble one of the predicted ratios of cross A or cross B.
   

2. How do you explain any differences between your actual results and predicted results? Is the difference significant?
   

   Actual results are a matter of probabilities, not certainties. Allele pairing is a matter of chance. Results are likely to reflect the predicted 9:3:3:1 or 1:1:1:1 ratios. A Chi Square test can determine if differences are mathematically significant.
   

3. Examine the genotypes in cross A above.

   What is the ratio of purple to non-purple? 3:1
   
   What is the ratio of smooth to wrinkled? 3:1
   
   Explain what the term independent segregation might mean.
   
   The alleles for color segregate during meiosis independent of the alleles for smooth and wrinkled. Thus the 3:1 is maintained for each and imbedded in the 9:3:3:1 ratio for the combined phenotypes.

Introduction

As we all know, it is common to find yellow corn kernels. Purple corn kernels are not nearly as common! The study of corn kernel color illustrates colored basic genetic principles.

Concepts

• Dominant/recessive
• Phenotype/genotype
• Dihybrid cross
• Independent segregation

Background

Each corn kernel on an ear of corn is a separate offspring resulting from a cross between a female parent (of which the ear and the corn cob are a part) and a male parent, which supplied the pollen grains that fertilized the egg. The fertilized egg then grew into the corn kernels (seeds). The colored layer of the seed that is visible is a part of the endosperm. If the endosperm is colored, it is observable. If the endosperm is not pigmented (colorless), the yellow color of the inner tissue shows through, and the color appears as yellow, white or cream.

In addition to color, kernels also display another easily observable trait. Those kernels that are starchy inside remain smooth when dried. Those kernels that contain sugar and are sweet, wrinkle when dried. Thus the kernels (offspring) can be purple and smooth, purple and wrinkled, yellow and smooth or yellow and wrinkled. (Note: Please consult a basic biology text for further details about the structure of a corn seed and the life cycle that produces seeds in plants.)

In producing the ears of corn used in this laboratory, the plant breeder started by selecting plants that were pure-breeding (i.e., plants whose offspring always produced purple, smooth kernels and another whose offspring always produced cream-colored, wrinkled kernels). The pollen from one of these strains was placed on the corn silks of the other strain. The seeds were then produced and represent the first generation of corn seeds. The genetic cross can be diagrammed as follows:

The plant breeder next made two kinds of crosses that produced the ears of corn used in this laboratory. The first cross involved crossing two of the first generation offspring (RrSs x RrSs). The second cross (often called a back cross) involved crossing a first generation offspring with one of the original non-purple offspring (RrSs x rrss).

Materials

Safety Precautions

Use caution when working with the marker pins. Do not pierce the corn kernels with the pins—push them into the soft corn cob tissue. Do not remove any corn kernels from the ear.

Procedure

1. Obtain a Corn Genetics Worksheet and complete the Predictions Section of the worksheet. If necessary, review key genetic terms such genotype, phenotype and phenotypic ratio, before attempting the predictions.
2. Obtain an ear of corn of unknown parentage. Do not remove any kernels from the ears. Count and record the number of each of the four phenotypes on the ear of corn. Put a colored starting marker pin on the end of one row of kernels in the soft corn cob tissue and then count and record the phenotypes in that row. The colored marking pin should remain in the corn until all the kernels are counted. One team member can announce each phenotype as another member tallies the count. Put an uncolored marking pin in the end of the next row and continue counting. After each row is completed, move the uncolored row marker pin to the next row until you return to the row marked by the colored pin.
3. When finished counting, record the total number of each phenotype on the Corn Genetics Worksheet in the Actual Count section.
4. Calculate the phenotypic ratio and then answer the questions on the worksheet.
5. All materials from this activity can be reused. Consult with your instructor for appropriate disposal and return procedures.

Student Worksheet PDF

10248_Student1.pdf