BI102
Name:
Genetics in Corn
Introduction: The field of genetics focuses on the study of how DNA-based information is passed from one
generation to the next. Gregor Mendel pioneered the field of genetics with his studies of inheritance in the
common garden pea. Mendel worked meticulously, moving tiny grains of pollen from the flower of a specific
plant to the stigma (female receptacle) of a different plant. Over many years and many generations of garden
peas, Mendel devised his Theory of Inheritance. This theory can be broken down into five major parts:
1. Genetic variation is the result of the presence of different versions (alleles) of genes.
2. Offspring have two copies of every gene, one from each parent.
3. If the two copies of a gene present in an individual are different, only one version of the gene will be
visible in the appearance (phenotype) of the offspring. This allele is known as the dominant allele and
the other allele is called the recessive allele.
4. During gamete formation (meiosis), the two copies of the gene present in an individual separate into
different gametes (known as the Law of Segregation).
5. When examining more than one gene, each allele of each gene will separate into gametes
independent of the how the alleles of other gene(s) separate (known as the Law of Independent
Assortment).
Mendel’s work did not start or end with the garden pea. In the course of his studies, Mendel tried mice (until
the Bishop found the basement overrun with the products of Mendel’s experiments) and several other species
of plants. Since Mendel’s time, scientists have adopted several species, both plant and animal, as model
organisms for the study of genetics. All of these organisms reproduce quickly and proficiently, have a large
amount of obvious genetic diversity, and allow controlled crosses (the scientist can decide who mates with
whom). Most of these organisms are small so that many individuals can be bred in the small space of a
laboratory.
The goal of this lab is to give you the opportunity to walk in Mendel’s footsteps. Instead of the garden pea, we
will use a variety of corn. Zea mays (maize) is one of the most important food crops in the world. Large
numbers of scientists are constantly examining the structure, function, development and genetics of this plant.
Maize exhibits a variety of inheritable phenotypes throughout its life cycle. In today’s lab, we will look at
phenotypes that are displayed in corn seedlings and in corn seeds.
A. Monohybrid Crosses: We will begin by looking at a simple monohybrid cross of alleles that can be viewed
in young corn seedlings. This type of cross follows one trait (one gene).

Observe the flats of F2 seedlings provided for your class. Answer the following questions:

What trait do you think we are following in this cross?

What are the two different alleles of this trait?
The production of the seeds that produced the individuals you are observing is a bit tricky. It turns out that the
albino allele is fatal. That means that they never grow long enough to produce their own offspring. That means
that, when working with these traits, we cannot do a cross between two parents (a homozygous green plant
and a homozygous white plant). There was no P generation! If we were able to cross the P generation we
would find that the resulting offspring (the F1 generation) were all green.
 Why do you think that the albino allele is fatal?

Why is the color of the F1 generation important in this cross?
Instead of crossing true-breeding parent plants, scientists studying these alleles use special crosses to
determine if a green plant is heterozygous or homozygous. Then, they cross two heterozygous plants (we can
call these the F1 generation) to produce the F2 generation that you are observing. Now that we know a bit
about these plants, we can start doing the genetics. Let’s begin by diagramming the cross.



Assign symbols to the traits being observed in the cross
(complete Table 1). Things will run more smoothly if every
member of your group uses the same symbols!
Use these symbols to complete Figure 1, which diagrams the
cross that produced the plants we are looking at.
What do we know about the F1 generation? As stated above, the
F1 plants display the green phenotype. Can you determine the F1
genotype?
Table 1: Key to Symbols
Used for Leaf Color Alleles
in Zea mays
Allele
Symbol
Green
leaves
Albino
leaves
Figure 1: Diagram a monohybrid cross
F1 Offspring Phenotype: Green
Genotype:
F1 Offspring Phenotype: Green
X
Possible Gametes:
Genotype:
Possible Gametes:
F2 Offspring
Possible Phenotypes:
Possible Genotypes:
When performing any scientific experiment, even a genetic cross, you start with a hypothesis. The usual
starting point for genetics is the hypothesis that a particular gene follows Mendel’s laws. We know that genes
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that follow Mendel’s laws act in a certain way and can use that to predict what the offspring of the cross will
look like.
Hypothesis : The leaf color gene follows Mendel’s laws of inheritance.
Once we have a hypothesis, we can make a prediction of what the results should be.

What is your prediction? Finish the statement below:
If the leaf color gene follows Mendel’s laws, ______% of offspring of a cross
between a two F1 plants should be green and _______% of the offspring should
be albino.
Data Collection. Count the number of each phenotype present in the corn seedlings. Record your data in
Table 2. Calculate the % of the population displaying each allele.
Table 2. Leaf color in F2 offspring
Phenotype
Number of Seedlings
Counted
Genotype(s)
Percent of Total
Offspring Counted
Green
Albino
Total

100%
Does your data support your hypothesis? Explain your reasoning.
B. Dihybrid cross:
A dihybrid cross follows the inheritance of two traits (two genes). The F2 plants
produced by a dihybrid cross will show four different combinations of phenotypes. This time we will look at
phenotypes present in another part of the corn plant, the cob. The average corn plant produces a single ear of
corn with around 500 kernels per ear arranged in discrete rows. Each kernel represents a single corn seed (so a
single new and genetically unique individual). Corncobs are a great way to look at genetics because the large
number of individuals present in a single ear remains pretty contained in the cob. In this example we will look
at two traits in corn kernels: kernel color and kernel structure.

Obtain a parental cross card and an ear of F2 corn.
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Each side of the parental cross card shows the phenotypes of the two different alleles for a particular corn
kernel gene as displayed by members of the P generation (labeled P). The image labeled F1 shows the kernels
resulting from the cross between the members of the P generation. Remember that corn kernels are the
offspring of the cross even though they have not germinated into seedlings yet!

What is the genotype of the P generation of both genes on the parental cross card (homozygous or
heterozygous)?

What is the genotype of the F1 generation for both genes on the parental cross card (homozygous or
heterozygous)?

Which allele is dominant for each trait? Explain how you know.

Complete Table 3 by deciding on symbols for each allele. Remember that your work will be less
confusing if everyone in your group uses the same symbols!
Table 3: Key to Symbols Used for Kernel Color and Structure Alleles in Zea mays
Trait: Kernel Color
Symbol
Trait: Kernel Structure
Purple
Smooth
Yellow
Rough
Symbol
In this exercise we will look at both of these genes at the same time (a dihybrid cross). When you describe the
phenotype of an individual, you should include two alleles (one from each gene). The genotype of each
individual should contain four letters (AaBB). Gametes have one copy of each allele and should include two
letters (Ab).

Use your symbols from Table 3 to complete Figure 3.
Figure 3: Diagram of the Dihybrid Cross (Creating the F1 generation)
Parent Phenotype:
Parent Phenotype:
Yellow, smooth kernels
Purple, rough kernels
X
Genotype:
Genotype:
Possible Gametes:
Possible Gametes:
F1 Offspring Genotype:
F1 Offspring Phenotype:
The kernels on the F2 corncob represent the offspring of a cross between two F1 plants. Each kernel is a
member of the F2 generation.
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
Complete Figure 4. When you get to the bottom box in the figure use the cob to determine the four
possible phenotype combinations produced by this cross.
Figure 4: Diagram of the Dihybrid Cross (Creating the F2 generation)
F1 Offspring Phenotype
(from bottom of Fig. 3):
F1 Offspring Phenotype
(from bottom of Fig. 3):
X
Genotype:
Genotype:
Possible Gametes:
Possible Gametes:
F2 Offspring
Possible Phenotypes:
1.
2.
3.
4.
Determining the possible genotypes in a dihybrid cross is a bit more challenging. A helpful tool for examining a
dihybrid cross is a Punnett Square (Figure 5).

Add the gametes you determined in Figure 4 to the sides of the Punnett Square in Figure 5. Write the
genotypes of the offspring small enough that you will also be able to write the phenotype in each box.
Remember:
 Gametes are haploid and should have only one copy of each letter you
are using to symbolize the traits!
 Offspring are diploid and should have two copies of each letter you
are using to symbolize the traits!
 The genotypes inside the squares of the Punnett Square are the
genotypes of possible offspring.
Figure 5: Punnett Square of the Dihybrid Cross
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
Use Table 3 to determine the phenotype of each offspring in the Punnett Square. Write the phenotype
below the genotype in each square.

How many phenotypes are possible in this cross? List them in Table 4.

Determine the percentage of the offspring that should exhibit each phenotype. Add this information to
Table 5.
Table 5: Results of Dihybrid Cross Based on the Punnett Square
Possible Phenotypes
Totals:
Number with this phenotype in
Punnett Square
Percent of Offspring Predicted to
have this Phenotype
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100
Now, we have everything we need to make our hypothesis regarding this cross!
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
What is the hypothesis? Return to the monohybrid cross if you need a hint!
Hypothesis:

What do you predict the outcome of the cross will be (write it out like we did for the monohybrid cross!)
Prediction:
Data Collection:
Note: Do not damage the corncobs in anyway (e.g. removing kernels, marking them with a pen or
sharp object)
1. Determine the number of kernels that display each combination of phenotypes.
a. An “X” has been marked at the end of one row of kernels at the larger end of the corn
cob. Use this as your starting row.
b. Cut a length of blue tape equivalent to the length of your corn cob.
c. Align the tape with the bottom of the row marked with the X.
d. Use the provided laboratory counters to record the phenotype of each kernel in that
row. This usually works best if one teammate reads the phenotypes and a second
teammate clicks the counter.
e. When you reach the end of the row, move the tape so it covers that row and lines up
below the next row. Count the next row.
f. Continue counting until you get back to the X marking the first row.
2. Record your data in Table 6.
3. Gather additional data from the other groups in your class.
4. Determine the percent of offspring represented by each phenotype combination (record in
Table 6).
Table 6: Results of the Dihybrid Cross
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Phenotype
combination:
Your
data
Group
2
Group
3
Group
4
Group
5
Group
6
Totals
Percent
Total #
plants:

How does your data compare to your prediction? Is your hypothesis correct? Explain.

Why do we look at the percentage of offspring of each phenotype rather than genotype?

In this lab, you were asked to gather data from several other groups before calculating your results.
Why would it be important to have a large pool of data for a genetics experiment?
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