Distinguish genotype frequency from phenotype frequency with a simple example.

• A. Genotype frequency is the proportion of tall individuals.
• B. Genotype frequency is the proportion of TT, Tt, tt in the population.
• C. Phenotype frequency measures gene expression levels.
• D. Genotype frequency and phenotype frequency are the same.

Answer: (B)

Genotype frequency is about how common each genotype is in a population: TT, Tt, and tt. For example, if in a group of 100 individuals there are 25 TT, 50 Tt, and 25 tt, the genotype frequencies are 0.25, 0.50, and 0.25. This is exactly what the statement describes—the proportion of the different genotypes present.

Phenotype frequency, on the other hand, looks at the visible traits, such as tall versus short. If tall is dominant, both TT and Tt produce tall, so the tall phenotype would be 0.75 and short would be 0.25 in this example. So the two frequencies measure different things: one counts genotypes, the other counts observed traits.

The other options mix these ideas or misdefine phenotype frequency. Genotype frequency isn’t simply the proportion of tall individuals, because tall includes more than one genotype. Phenotype frequency isn’t about gene expression levels, which is a cellular-level concept. And they are not the same thing, since genotype and phenotype distributions can differ.