GENETICS OF CORN
Students develop familiarity with genetic terminology, Punnett Squares, making
predictions and comparing to real data to deduce the genotypes of the parental cross in
segregating genetic corn seedlings.
Materials and Preparation
• Segregating Genetic Seeds (Albino corn, 3:1) Ward’s Natural Sciences
catalog #86 805 ($6.60 for 100 seeds); http://www.wardsci.com
• Containers to sprout seedlings (10 seedlings/container); can be recyclables such as deli
trays, etc.
• Potting soil
• Access to source of light – either a bright window or indoor bank of lights (fluorescent
or other).
Alternate method: Make drawings of the possible corn “plots”, 10 seedlings per page,
using the sample data provided.
Size of student group: pairs or larger, depending on resources available
Soak the seeds overnight in a large bowl of water. The next day, give each student group
10 seeds to plant. Moisten potting soil prior to planting. Seeds should be planted about 2
cm deep. Be sure to water after planting. Place planted seeds in a warm area (light not
needed until seedlings breach the surface of the soil). Be sure to keep the soil moist.
Seeds will sprout in about 5-6 days. Place in a lighted area. Analysis can be done when
seedlings are a couple of inches tall. The amount of time generally needed from soaking
the seeds to the analysis is around ten days.
The process can be sped up by a few days by placing the seeds in moist paper towels
inside sealed plastic bags. Seedlings should be planted as soon as the cotyledons are
visible.
To Do and Notice
Each student group retrieves their planted seeds and discusses the following questions:
Did all of the seeds sprout? Do all of the seedlings appear the same? If not, how do they
differ?
Student groups will tally the number of green and the number of albino seedlings in their
plot. Collect the class data on the board.
Tell the students that green pigment in the corn seedlings is dominant to albino, and
review the meanings of dominant, recessive, homozygous and heterozygous, genotype
and phenotype (see example below). By analyzing the class data, what can students
Karen E. Kalumuck Copyright 2008
Exploratorium Teacher Institute
deduce about the genotypes of the parent corn plants? (Hint: have the students write out
all possible combinations of alleles of the parent plants, and eliminate the possibilities.
You may wish to use Punnett squares to illustrate this process.
Example
Green is Dominant to albino. G = green allele; g = albino allele
Possible Genotypes of seedlings and their phenotypes: GG or Gg = green
gg = albino
List all possible genotypes of parent plants, independent of the data collected:
GG x GG
Gg x GG
gg x GG
GG x Gg
Gg x Gg
gg x Gg
GG x gg
Gg x gg
gg x gg
Students may use Punnett squares for each cross, to help them to calculate the predicted
phenotypic ratios of green to albino plants.
SAMPLE DATA
Plot Number
1
2
3
4
5
6
7
8
9
10
TOTAL
(total # seedlings
= 92)
Percent of Each
Out of total #
Seedlings
Ratio of green:
albino:
Number of Green
9
7
6
10
8
5
9
5
8
5
Number of albino
0
3
2
0
1
4
1
3
2
4
72
20
72 divided by 92 =
0.78
20 divided by 92 =
0.22
3.5
Karen E. Kalumuck Copyright 2008
Exploratorium Teacher Institute
1
Looking at the class data, which possible genotypes of the parent plants can be
eliminated? Since not all of the plants are green, any of the crosses that would yield only
green plants can be eliminated as possibilities (shown below in plain italics):
Since there are many green plants, we know that both parents can’t be homozygous
recessive, so we may eliminate that cross (underlined and plain):
GG x GG
Gg x GG
gg x GG
GG x Gg
Gg x Gg
gg x Gg
GG x gg
Gg x gg
gg x gg
To produce albino offspring, each parent must carry at least one recessive allele.
Using Punnett Squares to calculate the expected phenotypic ratios of each of the
possibilities, you would make the following predictions:
If the parent plants were both heterozygous: Gg x Gg, the expected ratio of green to
albino plants is: 3:1.
If the genotypes of the parental cross were Gg x gg (or the reciprocal cross gg x Gg) we
would predict a ratio of green: albino seedlings as 1:1.
Compare the predictions to the actual data. Which set of parental genotypes most closely
fits with our predictions? Can you predict the genotypes of the corn seedlings?
What’s Going On?
The segregating genetic corn seeds provide an optimal way for students to learn basic
genetic inheritance patterns (and the associated terminology and techniques) with plants
that they themselves have raised. By listing all possible genotypes and phenotypes,
students can analyze the class data to eliminate possible parental crosses, since not all
plants are green, nor are all of the plants albino. Each parent must carry at least one
recessive allele for albino seedlings to be present. Left with just two possible genotypes
of the parents, it’s a much easier matter to calculate predicted results using a Punnett
square and comparing to class data.
Karen E. Kalumuck Copyright 2008
Exploratorium Teacher Institute
Parental Genotypes
The seeds are guaranteed to have an approximately 95% germination rate, so not all will
sprout. Conditions of planting may also have an influence. The total number of
seedlings is used to calculate the ratios from the experimental data. In this example, the
experimental ratio of 3.5: 1 is closer to the predicted ratio of a cross between
heterozygotes, than it is to the predicted results from a cross with one heterozygote and
one homozygous albino. We can conclude that the parental cross was Gg x Gg.
NOTE: It would be impossible for an albino corn plant to set seed, as it would not
photosynthesize. The gene is maintained in the line as heterozygote plants. If the
seedlings are 5 – 6 inches tall at the time of analysis, students may also notice that the
albino plants seedlings are usually shorter than the green seedlings.
Phenotype of seedlings
We know that the genotype of the albino seedlings must be gg. But we cannot deduce the
genotype of an individual green seedling. In this case, we can predict that about one third
of the green seedlings is homozygous dominant (GG) and two thirds are heterozygotes
(GG) but for a definitive answer, the corn would need to be pollinated with a known
heterozygote; if albino seedlings result, we know that the seed was a heterozygote. If all
progeny plants are green, then it was homozygous.
After the Experiment
Allow students to “adopt” corn seedlings, take them home, and grow them. They are not
genetically modified (the albino gene is a natural mutation) and have been known to bear
fruit when mature).
Alternative Methods
• Providing drawn images that reflect data you would like students to collect saves the
time and expense of raising seedlings, and the same analysis can be done. However,
students do not develop a the same connection to the plants as when they are responsible
for the seedlings.
• If you teach several sections of the same class, data can be gathered from more than
one class, and with the larger sample size, the ratios will be closer to the predicted values.
Karen E. Kalumuck Copyright 2008
Exploratorium Teacher Institute