Final answer:
In wheat, two codominant alleles at a single locus yield three phenotypes: white, light red (intermediate), and dark red. This manifestation reflects codominance where both alleles are expressed, resulting in the correct answer option 3) White, light red, and dark red.
Step-by-step explanation:
In a strain of wheat with two codominant alleles segregating at a single locus to determine the color of the kernel, the three possible phenotypes are white, light red, and dark red. This is because codominance allows both alleles to express their traits simultaneously when present in a heterozygous individual, resulting in the intermediate light red phenotype. When the alleles are homozygous for the white or dark red trait, they present as their respective colors without blending. Therefore, option 3) White, light red, and dark red correctly identifies the three phenotypes present in this strain of wheat.
Codominance permits a simultaneous expression in the heterozygous state, unlike complete dominance where a dominant allele masks the recessive one in a heterozygous genotype. The given ratios align with a typical monohybrid cross where a dominant and recessive allele pair results in a classical 3:1 ratio of phenotypes in the F₂ generation. However, when dealing with codominant traits, we can observe a 1:2:1 ratio corresponding to the red, pink (light red), and white phenotypes in the F₂ generation, respectively. This reflects the genotypic ratios of a monohybrid cross involving codominant alleles (RR, Rr, rr).
Additionally, complications such as epistasis can alter the expected phenotypic ratios, as seen in LibreTexts™ notes. Here, epistasis occurs when a dominant allele masks the expression of another gene, changing the expected phenotypic outcome of certain cross-breeding experiments.