Science practice 3

A corn seed, or kernel, is made up of pericarp, aleurone, and endosperm layers. Figure 1.1 shows the basic anatomy of a corn seed. The endosperm layer may be yellow or white. The aleurone layer may be purple, red, or colorless. Unless the aleurone is colorless, the color of the aleurone layer masks the color of the endosperm layer.

Figure 1.1

Endosperm color is determined by a single gene with two versions, or alleles. A corn seed's specific combination of alleles, or genotype, determines the physical color of the endosperm (phenotype). Aleurone color is determined by the interactions of three independently assorting genes. A genotype that contains at least one aleurone color allele R and one aleurone color inhibitor allele C will produce a purple aleurone. A genotype that contains at least one R, one C, and two aleurone color modifier alleles pp will produce a red aleurone. All other allele combinations will produce a colorless aleurone.

Table 1.1 shows the phenotypes that result from all possible allele combinations for each of the four corn seed color genes.

TABLE 1.1 Corn Seed Cross

In a single ear of corn, each individual kernel is a separate seed representing an independent outcome from the cross of the parental corn. This means that individual kernels on the same ear of corn can have different genotypes and phenotypes.

Students in a biology class examined several ears of corn that resulted from three different parental crosses. Students were told the parental phenotypes for each cross and were instructed to count the number of kernels of each color present on each ear of corn. Table 1.2 shows the students' kernel color data for each of the three crosses.

TABLE 1.2 Corn Seed Genetic Cross