Teaching Mendelian Genetics with Corn Ears Lab #8
A. Furlotte
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, the following corn ears are used as they are especially appropriate for
introductory activities. (For simplification purposes diagrams are included here, I
typically sue real corn kernels.)
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 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
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.
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 F 2 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).
Genetics problems:
1. Show the possible monohybrid crosses using all samples of the above ears of
corn. Please be sure to follow the guidelines for problem work and to include the
ratio’s, fractions and probability for the f1 generations.
2. Design 4 problems using the above alleles to represent a monohybrid, dihybrid,
incomplete dominance and a cross of unknown parents. Be sure to include an
answer key with your questions
3. Try your questions out on your group members and identify and write a journal
listing possible sources of error, any possible revisions and editing that were
necessary. Include your classmates opinions and feelings about the difficulty
level and expertise of your questions.