The Amazing Genetics of Corn
Background
Corn is an excellent organism for studying genetics for several reasons. There are many different phenotypes which are easy to identify in corn kernels (corn seeds). Also, an ear of corn produces many offspring (corn seeds), within a relatively short amount of time. Finally, because corn is an important food crop, its genetics have been extensively investigated. The two traits which will be studied in this lab are seed color and seed shape.
Each kernel on an ear of corn is a separate offspring resulting from a cross between a female parent (the ear and the corn cob) and the male parent, which supplied the pollen grains that fertilized the egg. Thus, ear kernel develops from a single fertilized egg! The colored layer of the seed that is visible is a part that is called the endosperm. If the endosperm is colored, it will be purple or red. If the endosperm is not pigmented (colorless), the seed will appear yellow, white, or cream. Another trait that is easily observable in corn is whether the endosperm inside the kernel remains starchy or sweet when it is dried. Kernels that have starchy endosperm are smooth and those that have sweet endosperm become wrinkled.
In order to produce the ears of corn that we will be working with in this lab, a plant breeder selected two pure-breeding parents. One parent always produced purple, smooth seeds and the other plant always produced cream-colored, wrinkled kernels. The seeds were then produced and represent the first generation of offspring.
Parents: RRSS X rrss
(Pure breeding (Pure breeding
purple and smooth) non-purple and wrinkled)
1st Generation: RrSr
All offspring purple and smooth)
The genetics breeder then made two kinds of crosses that produced the ears of corn used in this lab. The first cross involved crossing the first two generation offspring (RrSs X RrSr). The second cross was a test cross involving a cross between the first generation offspring and a non-purple and wrinkled offspring (RrSs X rrss).
Prelab
1. Write a reasonable, scientific hypothesis for this lab. (Hint: Which of the two crosses from the second generation do you think your corn cob contains?)
2. Prepare a data table for recording the number of corn kernels counted for each phenotype (observed results) and the number of corn kernels for each phenotype that you would expect the sample to have (expected results).
Procedure
1. Obtain an ear of corn from your teacher.
2. DO NOT remove the shrink wrap from the ear of corn!
3. DO NOT remove any corn kernels from the ear of corn!
4. Put a colored push-pen on the end of one row of kernels. DO NOT pierce the corn kernels; place the push-pen in the soft corn cob tissue.
5. Count and record the number of each phenotype of the corn kernels on your corn cob. One lab partner should observe and say the phenotype to their lab partner. The second lab partner should record tally marks for each phenotype observed.
6. Count at least 300 corn kernels!
7. Record your observed data for each phenotype in your data table.
8. Calculate the expected data for each phenotype and record the values in your data table.
Directions for Writing this Lab Report
5 pts HEADING: Write your heading in MLA style in the box on this paper and in the upper left corner of your loose-leaf paper.
10 pts NEATNESS: Write your lab report in BLUE or BLACK ink. Use a ruler to draw your data table. Do not scratch out work. If necessary, start over!
5 pts TITLE: Write a clear, scientific title for your lab report. You may not use the title at the top of this lab sheet.
5 pts SECTION HEADINGS: You will receive 1 point for each of the following section headings: Hypothesis, Data Table, and Analysis Questions.
5 pts HYPOTHESIS: Write a clear hypothesis for the corn cob that you were given.
30 pts DATA TABLE: Prepare a data table for recording the data that you obtained in this lab. Follow the instructions in the pre-lab section. Include a descriptive title for your data table. It is worth 5 points!
40 pts ANALYSIS QUESTIONS: Answer each of the following analysis questions in complete sentences and in your own words.
1. What are three reasons why corn is a good organism for studying genetics?
2. What is the genus and species name of corn? (Look this up on the Internet!) Make sure you write the genus and species name in the correct format (CH 18 p. 448 in your textbook will be helpful with this!)
3. Prepare a Punnett Square for the cross between 2 heterozygous parents (P+pS+s x P+pS+s). Make sure you include the phenotypes and ratios of the offspring.
4. Prepare a Punnett Square for the cross between 2 heterozygous parents (P+pS+s x ppss). Make sure you include the phenotypes and ratios of the offspring
5. Calculate the Chi Square of the observed verses expected values for the (P+pS+s x P+pS+s) cross.
6. Calculate the Chi Square of the observed verses expected values for the (P+pS+s x ppss) cross.
7. Was your hypothesis supported or rejected for the cross involving the two heterozygous parents (P+pS+s x P+pS+s)? (Hint: tie in your chi-square data to help you answer this question.)
8. Was your hypothesis supported or rejected for the cross involving (P+pS+s x ppss) cross? (Hint: tie in your chi-square data to help you answer this question.)
Table of Critical Values of X2p is the probability that the results could be due to chance alone. The numbers in parentheses below each value of p restate the chance: 9 in 10 is the likelihood that the results could be due to chance alone. In genetics studies, p values of 0.05 or greater are considered the result of chance. Values less than 0.05 are studied again to ensure the validity of the experiment.
Degrees of Freedom / p = 0.9
(9 in10) / p = 0.5
(5 in 10) / p = 0.2
(2 in 10) / p = 0.05
(1 in 20) / p = 0.01
(1 in 100) / p = 0.001
(1 in 1000)
1 / 0.158 / 0.455 / 1.642 / 3.841 / 6.635 / 10.827
2 / 0.214 / 1.386 / 3.219 / 5.991 / 9.210 / 13.815
3 / 0.584 / 2.366 / 4.642 / 7.815 / 11.345 / 16.268
4 / 1.064 / 3.367 / 5.989 / 9.488 / 13.277 / 18.465
5 / 1.610 / 4.351 / 7.289 / 11.070 / 15.086 / 20.517