A gene is a unit of heredity on a chromosome... alternate forms called alleles. Alleles for a particular gene occur Introduction

AP Lab #7 - Mendelian Corn Genetics
Introduction
A gene is a unit of heredity on a chromosome and can have
alternate forms called alleles. Alleles for a particular gene occur
in pairs and in sexually reproducing organisms each parent
contributes one allele to their offspring thus ensuring generic
variability. Alleles however can be expressed as dominant, and
are usually designated by a capital letter (for example, "B").
Alleles whose expression is masked by dominant alleles are
recessive, and are designated by a lower case letter (for
example, "b").
The genotype of an organism whether dominant (BB)
hybrid/heterozygous (Bb) or recessive (bb) includes all the
alleles present in the cell; and the manifestation for that
particular trait is called the phenotype. Corn is an excellent
choice for Mendelian inheritance studies because kernel color and texture is controlled by a single pair of alleles and the
gene for blue and smooth kernels is dominant over the yellow and wrinkled phenotype.
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 wrinkled (B), Yellow and Smooth (C), Yellow and Wrinkled (D); two conditions will be examined: A
monohybrid cross and a dihybrid cross.
Monohybrid Cross of Corn - A monohybrid cross begins with experimental breeding between two parents that breed
true for different forms of a single trait. The trait you will investigate in this problem is kernel color. The two forms of
kernel color we will look at are Purple (RR) and Yellow (rr). You will also be looking at kernel texture; smooth (DD) and
wrinkled (dd)
Dihybrid Cross of Corn - A dihybrid cross begins with experimental breeding between two parents that breed true for
two different traits. The trait you will investigate in this problem is kernel color and texture. The two forms of kernel
color we will look at are Purple wrinkled (RRDD), purple smooth (RRdd), yellow wrinkled (and Yellow, smooth and
wrinkled.
We start with a plant homozygous for purple kernels and cross it with a plant homozygous for yellow kernels. The
offspring that result from this cross are called hybrids and are the F1 generation. When two individuals from the F1
generation are crossed the offspring is called the F2 generation.
Background
In these activities, you will investigate several phenotypes of corn that are expressed in the seed. You will be given ears
of corn for your investigation. Each seed on an ear results from a separate fertilization event. Further, the seeds on the
ear are the F2 from a cross that began with two parental varieties of corn with contrasting phenotypes.
Materials:
• F1 generation Ear of corn (purple: yellow)
• F1 generation Ear of corn (smooth: wrinkled)
• F2 generation Ear of corn (purple: yellow, smooth:
wrinkled)
•
•
•
Corn Parental Cross Card A
Corn Parental Cross Card B
Transparency marker
Exercise A: The parental (P) and first filial (F1) generation
Your group will be given two Corn Parental Cross Cards, A and B, that show a cross of two parental varieties of corn with
contrasting phenotypes (as explained in the Introduction) as well as the F1 corn resulting from the cross. Two of you will
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work with the cross shown on card A and two will work with the cross shown on card B. All the P corn varieties are
homozygous for all the genotypes you will be studying.
Examine the cross that is shown on your Corn Parental Cross Card. Collect information and complete steps 1-5 below.
1. Will you treat this as a monohybrid cross or a dihybrid cross? Explain your answer.
_______________________________________________________________________________________________
_______________________________________________________________________________________________
2. Provide on a one-word description of the phenotype of each of your P corn varieties (you may use the ones provided
in the introduction section). Record the descriptions in the blanks below.
Phenotype of
A-1 ___________________________ B-1 - ___________________________
Phenotype of
A-2 ___________________________ B-2 - ___________________________
3. Provide the symbols to represent the alleles by which these phenotypes are inherited (you may use the ones
provided in the introduction section). Record the symbols in the blanks below.
Genotype of
A-1 ___________________________ B-1 - ___________________________
Genotype of
A-2 ___________________________ B-2 - ___________________________
4. Which allele is dominant? Explain how you know.
_______________________________________________________________________________________________
_______________________________________________________________________________________________
Exercise B: F1 and F2 Cross
Using the Punnett square below and information you recorded in Exercise A along with your knowledge of genetics;
record the results of the crossing between the F1 generation individuals from card “A”
Genotypes and expected ratios for the F2:
Below the Punnett square, give the phenotypes and their expected ratios for the F2 in card “A”
Phenotype
Ratio
__________________________
__________________________
__________________________
__________________________
__________________________
__________________________
Once again record the results of the crossing between the F1 generation individuals from card “B”
Genotypes and expected ratios for the F2:
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Below the Punnett square, give the phenotypes and their expected ratios for the F2 in card “B”
Phenotype
__________________________
__________________________
__________________________
Ratio
__________________________
__________________________
__________________________
5. What is the genotype of the F1?
_______________________________________________________________________________________________
_______________________________________________________________________________________________
To simulate the F2 generation; obtain ear of corn with the two contrasting phenotypes, purple: yellow and smooth:
wrinkled. Working in pairs; count and record in Table 1 and 2 (below) the number of grains of each phenotype; one
person should call out the phenotypes while the other records them in the table.
To make the count, begin with one row of kernels and using a transparency marker lightly mark the kernel while
recording the appropriate phenotype of each kernel in that row; for ease, place “tic” marks in the wide space in the
table below. Continue counting to ensure that you count for at least five rows. When finished counting, total your
results by providing the average from at least three group members.
Table 1: Observed phenotype count for F2 in card “A”
PURPLE
Phenotypes:
YELLOW
Team Count
Total:
Total:
Total for all the phenotypes counted:
Table 2: Observed phenotype count for F2 in card “B”
Phenotypes:
SMOOTH
WRINKLED
Team Count
Total:
Total:
Total for all the phenotypes counted:
6. What evidence do you have that the traits you are investigating are actually following Mendelian genetics
expectations? Could they simply be the result of environmental factors?
___________________________________________________________________________________________
____________________________________________________________________________________
____________________________________________________________________________________
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7. If the phenotypes are under genetic control, are they inherited through a single gene, a few genes, or many genes?
How do you know?
___________________________________________________________________________________________
____________________________________________________________________________________
8. Using the data from Table 1 & 2 and information from your Punnett square, calculate the expected counts for the F2
and record them below.
Table 1
Phenotype _______________ Expected count _______________
Phenotype _______________ Expected count _______________
Table 2
Phenotype _______________ Expected count _______________
Phenotype _______________ Expected count _______________
Are the deviations from the expected results for the phenotypic ratio of the F2 generation within the limits expected by
chance? To answer this question, statistically analyze the data using Chi-square analysis. Refer to the Chi- Square (x2)
Test section on Table 4 - Chi-Square Values and Probabilities. In the space below, calculate the Chi-square statistic for
the F2 generation (use it as an example for the rest of the calculations for this lab). Refer to Table 4 to determine the
probability that is associated with your x2 statistic.
X2 = Ʃ
Table 1
Kernel
Purple
Yellow
Totals
9. Chi Square Value =
10. Degrees of Freedom =
Total Observed (O)
X2 = Ʃ
12. Chi Square Value =
13. Degrees of Freedom =
Total Expected (E)
(O-E)2/E
_______
_______
11. Were the results obtained by chance alone?
Table 2
Kernel
Purple
Yellow
Totals
(𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 −𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂)2
𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸
Total Observed (O)
Yes
No
(𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸 −𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂)2
𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸
Total Expected (E)
_______
_______
14. Were the results obtained by chance alone?
Yes
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No
(O-E)2/E
Exercise C: A Different Cross
As a team, look again at the Corn Parental Cross Cards, “A” and “B”. Remember that all of the P seeds are homozygous.
Suppose you performed a new cross using, as your P corn varieties A-1 and B-1 only; working as a team, collect the
following information about this cross as you complete the steps below.
15. Give the genotypes and phenotypes for the P and F1 of this cross.
Parental
A-1
Genotype
B-1
Genotype
x
Phenotype
Phenotype
F1
Genotype
Phenotype
16. In the space below, construct a Punnett square showing the results of crossing your F1 to obtain an F2. Below the
Punnett square, give the phenotypes and their expected ratios for the F2 of this cross.
17. Phenotypes and expected ratios for the F2:
Phenotype
Ratio
_________________
__________________
_________________
__________________
Phenotype
__________________
__________________
Ratio
__________________
__________________
Obtain an F2 ear of corn with the purple: yellow smooth: wrinkled phenotypes; for this crossing exercise; count and
record phenotypes in Table 3 below (for this exercises you must count the entire ear of corn). Use the wide space in the
table to make “tic” marks and provide the average from at least three group members.
18. Table 3: F2 Phenotype Count for Purple/smooth and Yellow/wrinkled
Phenotypes
Purple/Smooth
Purple/Wrinkled
Yellow/Smooth
Yellow/Wrinkled
Team
Count
Total:
Total:
Total for all phenotypes counted:
Total:
5
Total:
19. Calculate the expected counts for the F2 and record them below.
Kernel
Total Observed (O)
Total Expected (E)
Purple
Yellow
Sweet
Starchy
Totals
(O-E)2/E
20. x2 = ____________
21. Were the results obtained by chance alone?
Yes
No
22. On the basis of the data recorded in Table 3, what new inference can you make about the inheritance patterns of
the phenotypes? (Are they linked? Why or why not?)
__________________________________________________________________________________________________
__________________________________________________________________________________________________
Table 4 - Chi-Square Values and Probabilities (5% or Less is Considered Significant)
In this table, note the column titled, "Degrees of Freedom." The degree of freedom is always one less than the
number of different phenotypes possible. For the monohybrid F1 in this experiment we have two possible
phenotypes so there are 2 - 1 = 1 degree of freedom. For a dihybrid F2, there are four possible phenotype
combinations and 3 degrees of freedom. The numbers to the right of the Degrees of Freedom column in the table
are X2 values.
Degrees of
Freedom
1
2
3
4
5
6
7
8
9
10
p=99%
95%
80%
50%
30%
20%
10%
5%
1%
0.000157
0.02
0.11
0.29
0.55
0.87
1.23
1.64
2.08
2.55
0.00393
0.10
0.35
0.71
1.14
1.63
2.16
2.73
3.32
3.94
0.064
0.44
1.01
1.64
2.34
3.07
3.82
4.59
5.38
6.17
0.45
1.38
2.36
3.35
4.35
5.34
6.34
7.34
8.34
9.34
1.07
2.41
3.66
4.88
6.06
7.23
8.38
9.52
10.6
11.7
1 .64
3.21
4.64
5.98
7.28
8.55
9.80
11.1
12.2
13.4
2.71
4.60
6.25
7.78
9.24
10.6
12.0
13.4
14.7
15.9
3.84
5.99
7.81
9.48
11.1
12.6
14.1
15.5
16.9
18.3
6.63
9.21
11.3
13.2
15.0
16.8
18.5
20.1
21.6
23.2
Results obtained not by chance (reject null hypothesis)
By chance
The percentages given at the top of each column represent the probability that the variation of the observed results
from the expected results is due to chance. If the probability value is greater than 5%, we accept the null hypothesis;
that is, our data was obtained by chance alone and did not meet or provide evidence of Mendel’s laws of
probabilities.
Example:
Image that Chi-Square X2 = 2.43 and from the table for 3 degrees of freedom; we can see that 2.43 is between 30%
and 50%. By statistical convention, we use the 0.05 probability level as our critical value. If the calculated chi-square
value is less than the 0.05 value, we accept the null hypothesis (that our results were obtained by chance). If the
value is greater than the critical value, we reject the null hypothesis. Therefore, because the calculated chi-square
value is greater than the critical value of 5%, we accept that the data fits Mendel’s dihybrid 9:3:3:1 ratio.
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