In roses, red flowers and long stems are dominant traits. A rose plant that is homozygous for both red flowers and long stems is crossed with a rose plant that is heterozygous for red flowers and homozygous for short stems. What percentage of the offspring will exhibit red, long-stemmed flowers?

Let R represent red flowers and r non-red flowers; let L represent long stems and l short stems. The first plant is RRLL, and the second is Rrll. Every offspring receives an R and an L from the first plant, so all offspring have red flowers and long stems, regardless of which alleles they inherit from the second plant. Therefore, 100 percent will exhibit both traits: choice D.

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