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Biology 22
Corn Genetics
For this exercise, you will investigate inheritance of kernel color in the corn Zea
mays. Epistatic interactions will be demonstrated by the outcome of three genetic
crosses, designated C, D and F. In each case, parental corn has been bred to
obtain an F1 generation. F1 individuals are crossed to each other to produce an
F2 generation. You will analyze kernel color for F2 ears of corn.
For an ear of corn, each kernel represents a separate outcome from the cross
between the parents. Each kernel therefore has its own genotype and
phenotype. Two elements that contribute to kernel color include the aleurone
layer and the endosperm. The aleurone layer is located just inside the outer
pericarp layer. The endosperm, a source of nutrition for the developing seedling,
is located below the aleurone layer. Endosperm color can be masked by
aleurone color since the aleurone layer covers the endosperm.
Several genes are involved in determining kernel color and the effects of their
alleles are outlined in the table below.
Trait
Endosperm Color
Aleurone Color
Aleurone Color
Inhibitor
Genotype
Y_
yy
R_
rr
C’_
CC or Cc
cc
Aleurone Color
Modifier
P_
pp
Phenotypic Effect
Yellow Endosperm
White endosperm
Purple Aleurone
Colorless aleurone
No aleurone color in the
presence of color alleles like R
Aleurone Color in the presence of
color alleles like R
Colorless aleurone layer in the
presence of color genes like R
No effect on aleurone layer
Changes purple aleurone to red
aleurone in the presence of R_
Relationship
Y>y
R>r
C’ > C > c
P>p
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For your analysis:
1. Obtain an F2 ear of corn and use adhesive dots to mark the
boundaries of a section containing 10 rows. A row is oriented from tip
to tip of the ear, in the longer direction. Be careful not to tear the
cellophane covering the ear.
2. Record the number of kernels of each color that are found within the
boundaries. Record your data directly on the accompanying worksheet
pages.
3. Predict the expected genetic ratio for each set of data and use a chisquare test to determine if your data fits the expected ratio.
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Biology 22: Corn Genetics Worksheet
NAME________________________
Cross C
Parental Phenotypes
Yellow
X
Yellow
Parental Genotypes
YYC’C’RR X
YYCCrr
F1 Phenotype
Yellow
F1 x F1 Genotypes
YYC’CRr
X
YYC’CRr
F2 Phenotypes
Yellow
Purple
Class Totals for Observed Numbers of
Kernels in F2 generation
In the space below, illustrate the expected genotypes for the F1 x F1 cross with a
Punnett Square. What is the expected phenotypic ratio for Cross C in the F2
generation?
Use a chi-square analysis to test the expected ratio for the F2 of Cross C.
Phenotype Observed Expected
Number
Ratio
Expected
Number
(O-E)
(O-E)2
(O-E)2/E
Total =
Chi-square
_______
Number of degrees of freedom=
Probability of a match=
On the basis of your chi-square test, does your data support the expected ratio?
If so, indicate the criterion you used to justify the support. If not, give a plausible
reason for the failure of the data to reflect the predicted ratio.
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Biology 22: Corn Genetics Worksheet
NAME________________________
Cross D
Parental Phenotypes
Parental Genotypes
F1 Phenotype
F1 x F1 Genotypes
F2 Phenotypes
Class Totals for Observed Numbers of
Kernels in F2 generation
Purple
yyPPRR
X
White
X
yypprr
Purple
yyPpRr
X
yyPpRr
Purple
Red
White
In the space below, illustrate the expected genotypes for the F1 x F1 cross with a
Punnett Square. What is the expected phenotypic ratio for Cross D in the F2
generation?
Use a chi-square analysis to test the expected ratio for the F2 of Cross D.
Phenotype Observed Expected
Number
Ratio
Expected
Number
(O-E)
(O-E)2
(O-E)2/E
Total =
Chi-square
_______
Number of degrees of freedom=
Probability of a match=
On the basis of your chi-square test, does your data support the expected ratio?
If so, indicate the criterion you used to justify the support. If not, give a plausible
reason for the failure of the data to reflect the predicted ratio.
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Biology 22: Corn Genetics Worksheet
NAME________________________
Cross F
Parental Phenotypes
Parental Genotypes
F1 Phenotype
F1 x F1 Genotypes
F2 Phenotypes
Class Totals for Observed Numbers of
Kernels in F2 generation
White
yyCCrr
yyCcRr
Purple
X
White
X
yyccRR
Purple
X
yyCcRr
White
In the space below, illustrate the expected genotypes for the F1 x F1 cross with a
Punnett Square. What is the expected phenotypic ratio for Cross E in the F2
generation?
Use a chi-square analysis to test the expected ratio for the F2 of Cross F.
Phenotype Observed Expected
Number
Ratio
Expected
Number
(O-E)
(O-E)2
(O-E)2/E
Total =
Chi-square
_______
Number of degrees of freedom=
Probability of a match=
On the basis of your chi-square test, does your data support the expected ratio?
If so, indicate the criterion you used to justify the support. If not, give a plausible
reason for the failure of the data to reflect the predicted ratio.