If true-breeding dark blue-flowered and true-breeding white-flowered plants are crossed, the hybrid plant has light blue flowers. If the progeny of this first cross is allowed to interbreed, what fraction of the next generation would be expected to have light-blue flowers?

The fraction of the next generation expected to be light blue-flowered plants is 1/2, that is 50% of the F2.

Explanation:

Incomplete dominance is a condition where neither of the alleles completely dominates over the other one. Dominant alleles cannot completely cover up the recessive alleles. Descendents possess an intermediate phenotype between the two parental phenotypes and not the dominant one, which would appear if this would be the case of complete dominance.

In the example, a pure breeding blue-flowered plant and a pure breeding white-flowered plant are crossed, producing a new generation of 100 % heterozygote light blue-flowered plants, an intermediate phenotype between both homozygotes.

When individuals of the first progeny interbreed, dark blue and white traits appear again. In this way, the next generation is 25% homozygote dark blue-flowered plants, 25% homozygote white-flowered plants, and 50% heterozygote light blue-flowered plants.