1 Name__________________________________________________ Date:_____________ Block:_____ Lab – Dihybrid Corn Genetics Background Before Mendel, it was thought that offspring characteristics were the result of blended traits received from each parent. Mendel, based on his observations while breeding pea plants and with his background in mathematics and statistics, hypothesized that hereditary material is passed as discrete packets from parents to offspring. Today we know that genetic information is coded in DNA on different chromosomes that are sorted and passed from parent to offspring. Genes located on the same chromosome are called linked. Genes located on different chromosomes are unlinked and assort independently of one another. Corn (Zea mays) is a large, coarse grass. Male and female flowers are separate (Figure 1). Many male flowers make up each of the tassels, which are located at the tips of stems. The tiny pollen grains are transported by wind. Female flowers nestle lower down, close to the stout part of the stem and are combined in structures we refer to as an ear. Each kernel in an ear of corn is the swollen ovary of a flower; a single strand of silk extends from each kernel to the outside of the ensheathing husk. A kernel only matures if its silk is pollinated. Thus each kernel in an ear of corn represents a separate fertilization of an ovum by a sperm. An ear of corn holds the results of hundreds of separate genetic crosses. As you know from class and your textbook, it is possible to predict the results of a cross involving parents who are heterozygous for two traits. In this investigation, you will use hybrid corn to study the inheritance patterns of corn kernel color and shape. In corn, purple kernels are the dominant to yellow kernels. Smooth kernels are dominant to wrinkled kernels. In this lab, you will use statistical methods to answer the following question: • Are the genes for kernel color and kernel texture most likely linked or unlinked? In other words, are they most likely found on the same chromosome or on different chromosomes? Figure 1: Structure of Zea mays Figure 2: Kernel phenotypes Purple & Smooth (A), Purple & Wrinkled (B), Yellow & Smooth (C) and Yellow & Wrinkled (D).
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Name__________________________________________________ Date:_____________ Block:_____
Lab – Dihybrid Corn Genetics
Background Before Mendel, it was thought that offspring characteristics were the result of blended traits received from each parent. Mendel, based on his observations while breeding pea plants and with his background in mathematics and statistics, hypothesized that hereditary material is passed as discrete packets from parents to offspring. Today we know that genetic information is coded in DNA on different chromosomes that are sorted and passed from parent to offspring. Genes located on the same chromosome are called linked. Genes located on different chromosomes are unlinked and assort independently of one another.
Corn (Zea mays) is a large, coarse grass. Male and female flowers are separate (Figure 1). Many male flowers make up each of the tassels, which are located at the tips of stems. The tiny pollen grains are transported by wind. Female flowers nestle lower down, close to the stout part of the stem and are combined in structures we refer to as an ear. Each kernel in an ear of corn is the swollen ovary of a flower; a single strand of silk extends from each kernel to the outside of the ensheathing husk. A kernel only matures if its silk is pollinated. Thus each kernel in an ear of corn represents a separate fertilization of an ovum by a sperm. An ear of corn holds the results of hundreds of separate genetic crosses.
As you know from class and your textbook, it is possible to predict the results of a cross involving parents who are heterozygous for two traits. In this investigation, you will use hybrid corn to study the inheritance patterns of corn kernel color and shape. In corn, purple kernels are the dominant to yellow kernels. Smooth kernels are dominant to wrinkled kernels.
In this lab, you will use statistical methods to answer the following question:
• Are the genes for kernel color and kernel texture most likely linked or unlinked? In other words, are they most likely found on the same chromosome or on different chromosomes?
Figure 1: Structure of Zea mays
Figure 2: Kernel phenotypes Purple & Smooth (A), Purple & Wrinkled (B), Yellow & Smooth (C) and Yellow & Wrinkled (D).
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Part I: Determining the Expected If the genes are located on DIFFERENT chromosomes, we would expect them to assort independently from each other according to Mendel’s Law. If the genes are located on the SAME chromosome, however, the predictions we make using a Punnett square would not be accurate. You will use a Punnett square to predict the results of a cross between two dihybrid F1 corn plants, assuming the genes are unlinked (not on the same chromosome) and thus assort independently in meiosis.
1. Draw a Punnett square that shows expected genotypic and phenotypic results from crossing 2 dihybrid corn plants that are heterozygous for purple and smooth kernels. Please use the assigned letters for each allele.
A= purple a= yellow B= smooth b= wrinkled.
2. Using the results of your Punnett Square, complete the following Expected Phenotype table:
Table 1: Expected Phenotypes Phenotypes Phenotypic Ratios
purple, smooth
/ 16
purple, wrinkled
/16
yellow, smooth
/16
yellow, wrinkled
/16
TOTAL
16 / 16
3. What is the expected Phenotypic Ratio? ____: ____: ____:____ 4. Hypothesize: How will the observed phenotypic ratios compare to the prediction you made in #3?
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Part II: The Actual Observed Outcome
5. Obtain an ear of dihybrid corn. 6. Observe and record the phenotypes of 4 rows of kernels in Table 2 below.
a. One person should call out the phenotypes for each kernel in the row, while another person records tally marks in the appropriate cells in the data table.
b. Continue this process for 4 continuous rows of corn (total kernels should be 150-‐200). c. Record class data in Table 2.
Table 2: Data Table for a Dihybrid Cross Observed Outcomes
Phenotype Number Counted (o) GROUP
Number counted (o) CLASS
Purple smooth
Purple wrinkled
Yellow smooth
Yellow Wrinkled
Total
7. What is the observed phenotypic ratio based on class data? ____: ____: ____:____ 8. Is your ratio close to the expected ratio of 9:3:3:1 ratio? Explain.
Part III: Determining Significance of Data
What is the 𝑿𝟐 test? The Chi-‐Square Test calculates two statistical measures:
1. The deviation between your observed numbers and your expected numbers. 2. The probability that the deviation is due to chance or that the deviation is statistically significant.
If the deviation between your expected and observed phenotypic ratios is found to be due to chance (p-‐value greater than 5%), then your results support (fail to reject) the null hypothesis that no significant difference exists between the expected and observed values. If the deviation between your expected and observed phenotypic ratios is found to be less than 5%, then you reject the null hypothesis and conclude that the difference between the observed and expected data is likely caused by something other than chance.
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The 𝑿𝟐Method Step 1: Determine the ratio of phenotypes and the number of kernels of each phenotype you expected.
Phenotype Expected Ratio Expected Number of Kernels (“e”) out of class totals.
1.
2.
3.
4.
Step 2: Record the ratio of phenotypes and the number of kernels of each phenotype you observed.
Phenotype Observed Ratio Observed Number of Kernels (“o”) out of class totals.
1.
2.
3.
4.
Step 3: Complete the table using class data. For “o” and “e” values, use the actual numbers of corn kernels rather than ratios.
The formula for the chi-‐square test is: 𝑋! = {(!!!)!
!} Where
𝑋!= chi square value o= observed results e = expected results. Σ = sum
Table 3: Data Table for a Dihybrid Cross Observed Outcomes
Expected Outcome (out of class totals )
(𝑜 − 𝑒)!
𝑒
Phenotype Count (o) Expected (e) Purple smooth
Purple wrinkled
Yellow smooth
Yellow wrinkled
Chi Squared Value! Add the numbers from the rows above.
Step 4: Calculate the degrees of freedom by subtracting 1 from the number of possible phenotypes.
Possible phenotypes = _______________
Degrees of freedom = _______________
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Step 5: Decide whether to accept or reject your hypothesis. Using the table below, determine the probability that the deviation between the expected results and observed results is due to chance. The numbers below the percentages are the chi-‐squared values.
Chi-‐Square Distribution Table
Degrees of Freedom
Probability of a Chance Occurrence
90% 70% 50% 30% 20% 10% 5% 1%
1 0.016 0.148 0.455 1.074 1.642 2.706 3.841 6.635
2 0.211 0.713 1.386 2.408 3.219 4.605 5.991 9.210
3 0.584 1.424 2.366 3.665 4.642 6.251 7.815 11.341
4 1.064 2.195 3.357 4.878 5.989 7.779 9.488 13.277
Probability (P) Value = ____________
If the probability value is 5% or greater, then your results support the null hypothesis that there is no difference between the observed and expected values and/or if there is a slight difference, this difference is caused by chance and chance alone. If the probability is less than 5%, then your results do not support the null hypothesis that the difference is due to chance. Instead, you reject the null hypothesis and conclude that the difference between your results is not due to chance; the difference is statistically significant.
Post Lab Questions
1. Is the observed outcome consistent with the expected outcome? Explain your answer using your data as evidence.
2. Are the genes for kernel color and kernel texture most likely “linked” (on the same chromosome) or “unlinked” (on separate chromosomes)? Explain your answer using your knowledge of meiosis and your data as evidence.
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