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Corn Genetics Lab
A gene is a unit of heredity on a chromosome and can have alternate forms called alleles. In
sexually reproducing organisms each parent contributes one allele to their offspring that may or
may not be like the other parent's allele. Alleles for a particular gene occur in pairs.
Alleles that mask expression of other alleles of a particular gene, but are themselves expressed
are 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 includes all the alleles present in
the cell, whether they are dominant or recessive. The
Biochemical manifestation of the trait is called the phenotype.
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 Blue and Yellow. Corn is a good organism for this
type of analysis since each grain (kernel) represents an
independent offspring.
We start with a plant homozygous for blue 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. By observing
the progeny from our crosses, we can form a couple of
hypotheses.


Kernel color is controlled by a single pair of alleles.
The gene for Blue kernels is dominant.
One important fact to remember: Despite what you may think,
most traits are not determined by single pairs of alleles. We can examine whether kernel color
is truly a "single gene trait" by statistically comparing the results of our crosses with those that
would be expected from a trait controlled by a single pair of alleles. One method to perform this
type of analysis is called Chi square analysis (Χ2).
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The formula for Chi square analysis is shown below. Don't let this simple math confuse you just
because there are symbols in an equation. We will practice this once before you go to lab.
Χ2 (chi squared) - This is just
the name of the analysis.
Σ- This is an operator that
says to sum all the values to
the right.
o - These are values you
measure or observe.
e - These are values you
expect. In chi square analysis
you test o vs e.
Why is corn an excellent choice for introducing students to Mendelian inheritance? To start
with, we do the crosses and supply F2 ears of corn, so you can literally do an instant genetics
lab. There are numerous contrasting phenotypes expressed in corn kernels (corn seed). These
phenotypes are easy to recognize, so even beginners can score them with confidence. Also, an
ear of corn has many kernels, making it a convenient unit for classroom use. And finally,
because corn is an important food crop, its genetics have been extensively investigated. To help
you introduce corn genetics in your class, I have highlighted a few of our corn ears that are
especially appropriate for introductory activities. But before we move on to specific examples,
let's discuss the phenotypes studied in these activities: kernel color and endosperm
characteristics.
Kernel color
Many genes determine the phenotypes of the 3 tissues that control the color of a corn kernel.
These tissues are the pericarp, the aleurone (outer layer of the endosperm), and the
endosperm proper. In our corn, the pericarp is always colorless, but the aleurone may be
colorless, purple, or red, and the endosperm yellow or white.
If the aleurone is colorless, the kernel color will be that of the endosperm, either yellow or
white. Normal corn endosperm color (yellow) occurs when the allele Y causes the production of
carotenoid pigments in the endosperm. In the recessive condition (y/y) carotenoids are not
produced and the endosperm is white. The Y alleles are masked by the presence of a colored
aleurone.
Figure 1 The tissues of a corn kernel involved in producing color phenotypes.
For the aleurone to be colored, alleles C and R must be present. The homozygous recessive of
either allele (c/c or r/r) disrupts anthocyanin production and results in a colorless aleurone. The
dominant CI allele also inhibits anthocyanin production, giving a colorless aleurone. Genes C and
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R are located on separate chromosomes and segregate independently.
The allele Pr interacts with alleles C and R to produce a purple aleurone. The homozygous
recessive condition (pr/pr) interacts with C and R to produce a red aleurone.
Endosperm characteristics
Normal corn endosperm is high in amylose starch. The gene Su in the homozygous recessive
condition (su/su) produces endosperm that is high in sugar. As corn dries, its sugary endosperm
loses water, and its kernels wrinkle. The gene Wx in the homozygous recessive condition
(wx/wx) causes the production of amylopectin starch in the endosperm and pollen. The
endosperm of a wx/wx kernel is opaque with a hard, waxy texture.
Figure 2 Purple starchy
Figure 3 Yellow starchy
Figure 4 Yellow sweet
Figure 5 Purple yellow cross
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Figure 6 Starchy sweet cross
Figure 7 Cross of R and SU alleles
Figure 8 Pr, C, and R alleles
The crosses
Students begin by studying F2 ears from 2 monohybrid crosses: a cross of purple starchy (R/R
Pr/Pr Y/Y Su/Su) with yellow starchy (r/r Pr/Pr Y/Y Su/Su) or a cross of yellow starchy (r/r Pr/Pr
Y/Y Su/Su) with yellow sweet (r/r Pr/Pr Y/Y su/su). See Figures 2, 3, and 4.
Example 1: In our first example, R Color Alleles 3:1, only the R color alleles undergo
segregation and recombination, so we show only those alleles, while remembering that the
other alleles (Pr, Y, and Su) are present.
P1 R/R x r/r
F1 R/r heterozygous purple
F1 cross R/r x R/r
F2 R/R R/r r/r phenotypes Purple:Yellow in a phenotype ratio of 3:1
Example 2: In our second example, Su Endosperm Alleles 3:1, only the Su alleles are
segregating and recombining.
P1 Su/Su x su/su
F1 Su/su heterozygous starchy
F1 cross Su/su x Su/su
F2 Su/Su Su/su su/su phenotypes Starchy:Sweet in a phenotype ratio of 3:1
Figure 5 shows part of an F2 ear of the purple yellow cross, and Figure 6 shows a similar view
of an F2 ear of the starchy sweet cross. Students can score the phenotypes of the kernels and
calculate phenotype ratios. They can then calculate X2 to see if their data fits expected values.
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Example 3: Now challenge your students to predict the genotypes and phenotype ratios of the
F2 that would result from a cross of the purple starchy and yellow sweet parental types. They
should realize that this is a dihybrid cross.
P1 R/R Su/Su x r/r su/su
F1 R/r Su/suheterozygous purple heterozygous starchy
F1 cross R/r Su/su x R/r Su/su
F2 phenotypes Purple Starchy:Purple Sweet:Yellow Starchy:Yellow Sweet in a 9:3:3:1 ratio
(Figure 7).
Students can check their predictions by scoring the phenotypes of kernels on an F2 ear of this
cross (Figure 7, R and Su Alleles 9:3:3:1) and calculating ratios and X2. Since they have
previously used F2 ears from the monohybrid crosses, it should be easy for students to see that
this dihybrid F2 ear is simply a combination of 2 monohybrid crosses. You can reinforce this by
having students compare the numbers of purple to yellow kernels and starchy to sweet kernels
in the dihybrid F2 ear. They should find phenotype ratios of 3:1 as in Examples 1 and 2.
Challenge students to explain how the two 3:1 ratios combine to give the 9:3:3:1 ratio of the
dihybrid.
Gene interactions
The reason we use phenotypes like purple and yellow in introductory genetics is that their
inheritance follows apparently simple patterns. Most phenotypes result from the interactions of
several genes. Corn offers many examples of gene interactions in both monohybrid and dihybrid
crosses. Here is an example that produces an interesting alteration to the standard dihybrid
9:3:3:1 phenotype ratio.
Example 4: This example is an F2 ear that begins with a cross of two white corns and involves
interactions of the Pr, C, and R alleles. Both parents are homozygous (y/y) for white
endosperm. If either color allele is present as a homozygous recessive, c/c or r/r, the kernel will
be white. Both alleles must be present as homozygous or heterozygous dominants for the
kernel to be colored. Both parents are homozygous pr/pr, which gives a red as opposed to a
purple kernel. Again, we show only the alleles that undergo segregation and recombination.
P1 C/C r/r x c/c R/R
F1 C/c R/r
F1 cross C/c R/r x C/c R/r
F2 phenotypes Red, White in a 9:7 ratio (Figure 8).