In this exercise, you will examine an ear of corn and determine the type of cross and genes responsible for the coloration and texture of the corn kernels like the one show below. There are four grain phenotypes in the ear. Purple and smooth (A), Purple and Shrunken (B), Yellow and Smooth (C), Yellow and Shrunken (D).
Monohybrid Cross
1. Count the number of purple and yellow kernels in five of the rows on your ear of corn and record the number on the chart. Be sure to use the same five rows for each calculation.
2. Count the number of smooth and shrunken seeds on the same five rows and record on the chart.
| Number of Kernels | Kernal Percentage (divide count by total) |
3. What are the probable phenotypes of the parents with regard to coloration?
4. What are the probable phenotypes of the parents with regard to texture? |
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| Kernal Coloration | |||
| Purple |  |
|
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| Yellow | |
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| Total (for 5 rows) | |
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| Kernal Texture | |||
| Smooth | |
|
|
| Shrunken | |
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| Total (for 5 rows) | |
|
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Dihybrid Cross
5. We will now consider a dihybrid cross, which is a combination of the two monohybrids. Your ear of corn may be a result of a cross between plants that were both heterozygous for color and texture (Pp x Ss). Work out this cross in the Punnet square below.
6. Calculate the phenotypic ratios for each type of seed.
Purple & smooth _______________
Purple & shrunken ______________
Yellow & smooth _______________
Yellow & shrunken ______________
7. Now count the number of each in your five rows on the ear of corn.
| Number Counted | Ratio: Number counted / total | |
| Purple & smooth | ||
| Purple & shrunken | ||
| Yellow & smooth | ||
| Yellow & shrunken | ||
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TOTAL
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8. Did you obtain a 9:3:3:1 ratio? If you did not, then the genes may be found on the same chromosome and do not assort independently. To determine if the deviations from your observed data are due to chance alone or if the data is significantly different, you need to use a chi square test.
First calculate the expected
number you should have gotten based on your total number assuming a 9:3:3:1
ratio.
Calculate the individual chi square values for each row and add them all together
to determine your overall chi square value.
| Expected Number | Observed Number | 
÷ expected |
|
| Purple & smooth | Total x 9/16 = | |
|
| Purple & shrunken | Total x 3/16 = | |
|
| Yellow & smooth | Total x 3/16 = | |
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| Yellow & shrunken | Total x 1/16 = | |
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CHI SQUARE VALUE ========> (add the numbers from
the rows above) |
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9. Now determine if your chi square value is a good fit with your data. Your degrees of freedom (df) is the number of possible phenotypes minus 1. In your case, 4 - 1 = 3. Find the number in that row that is closest to your chi square value. Circle that number.
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Good
Fit Between Ear & Data
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Poor
Fit
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df
|
.90
|
.70
|
.60
|
.50
|
.20
|
.10
|
.05
|
.01
|
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1
|
.02
|
.15
|
.31
|
.46
|
1.64
|
2.71
|
3.85
|
6.64
|
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2
|
.21
|
.71
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1.05
|
1.39
|
3.22
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4.60
|
5.99
|
9.21
|
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3
|
.58
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1.42
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1.85
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2.37
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4.64
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6.25
|
7.82
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11.34
|
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4
|
1.06
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2.20
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2.78
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3.36
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5.99
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7.78
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9.49
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13.28
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10. Explain what it means to have a "good fit" or a "poor fit". Does you chi square analysis of real corn data support the hypothesis that the parental generation was PpSs x PpSs?
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PROBLEM SET Chi Square Problem: A large ear of corn has a total of 433 grains, including 271 Purple & starchy , 73 Purple & sweet, 63 Yellow & starchy, and 26 Yellow & sweet. These numbers are entered in Columns 1 and 2 of the following Table 4. Your Tentative Hypothesis: This ear of corn was produced by a dihybrid cross (PpSs x PpSs) involving two pairs of heterozygous genes resulting in a theoretical (expected) ratio of 9:3:3:1. Objective: Test your hypothesis using chi square and probability values. SHOW ALL WORK!
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