What is the result when a plant with red flowers (RR) is crossed with a plant with yellow flowers (YY), assuming the alleles are codominant?

The red-flowered plant can pass on only an R allele, and the yellow-flowered plant can pass on only a Y allele, so their offspring will all have the genotype RY. With codominance, both alleles are expressed distinctly rather than blending together. The offspring will therefore have a mixture of red and yellow petals, choice D.

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