Final answer:
The dominance of an allele does not necessarily equate to its prevalence in a population. Most people have five fingers because the polydactyl allele is very rare. In isolated populations like the Amish, the allele can be more common due to increased chances of both parents carrying the dominant trait.
Step-by-step explanation:
The question involves understanding why, despite polydactyly being a dominant genetic trait, it is not the prevailing characteristic in the human population. When we discuss polydactyly, particularly postaxial polydactyly in Caucasian populations, we are referring to the genetic trait where individuals have an extra digit, typically a sixth finger, on one or both hands. This trait is often seen in communities with less genetic diversity, such as the original Amish colonies, where it appeared with a higher frequency due to the founder effect.
Answering the provided multiple-choice question, most people have five fingers because:
- The polydactyl allele is very rare in the human population.
This rarity is due to the fact that although the allele for polydactyly is dominant, it is not widespread across the global population. Instead, it occurs in low frequencies; therefore, five fingers is the more common trait. Inbreeding within isolated communities, such as some Amish populations, has led to a higher manifestation of polydactyly due to an increased likelihood of both parents carrying the rare dominant allele.