Which type of dominance is the problem in testing qualitative traits?

• A. Complete dominance
• B. Incomplete dominance
• C. Codominance
• D. Epistasis

Answer: (B)

Qualitative trait testing relies on clear, discrete phenotypes that map neatly to genotypes, so you can infer the genetic makeup from what you observe. When one allele is completely dominant, the heterozygote looks like the dominant homozygote, keeping things simple: two distinct phenotypes and straightforward Mendelian predictions.

In incomplete dominance, the heterozygote expresses an intermediate phenotype that is not identical to either homozygote. This blurs the link between phenotype and genotype, so a single observation doesn’t reliably reveal which genotype is present. For example, crossing red and white flowers can yield pink heterozygotes, so you don’t have a clean binary classification to guide selection or genotype inference. This blending of phenotypes makes it harder to classify individuals and predict outcomes, which is why incomplete dominance is the type that creates the problem for testing qualitative traits. Codominance still presents distinct phenotypes for different genotypes (just with both traits expressed), and epistasis involves interactions between genes, which are different kinds of complications.