Teaching Mendelian Genetics with Corn Ears
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.
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.
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.
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 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.
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).
Working out the genetics involved in such crosses can be challenging (and fun!) for students who are ready to move beyond the basics. To continue your class's exploration of corn genetics, see our printed or online catalog for a comprehensive selection of corn genetics kits and resources at: www.carolina.com/corn.