Final answer:
A testcross is not needed to determine if a snapdragon with red flowers is homozygous or heterozygous due to incomplete dominance, where heterozygous snapdragons exhibit pink flowers.
Step-by-step explanation:
A testcross is unnecessary to determine whether a snapdragon with red flowers is homozygous or heterozygous due to the phenomenon of incomplete dominance in its flower color inheritance. In snapdragons, a cross between a homozygous red-flowered plant and a homozygous white-flowered plant yields heterozygous offspring with pink flowers, indicating the expression of both alleles in an intermediate phenotype. Hence, a red-flowered snapdragon plant can only be homozygous, as a heterozygous plant would exhibit pink flowers, not red. This is why a testcross, which is typically used to discern between a homozygous dominant and a heterozygous dominant phenotype, is not necessary in this case.