If the allele encoding polydactyly (six fingers) is dominant why do most people have five fingers?
genetic elements suppress the polydactyl gene.
polydactyly is embryonic lethal.
the sixth finger is removed at birth.
the polydactyl allele is very rare in the human population.

The polydactyl allele is very rare in the human population.

Explanation:

The dominant alleles are expressed in both homozygous and heterozygous states. However, the harmful dominant alleles or the dominant alleles that do not impart the survival and/or reproductive advantages to the individuals exhibit lower frequency in natural populations.

The allele for polydactyly is dominant but is present in lower frequency in the natural human populations. Being a less frequency dominant allele, it is not expressed in most of the individuals. So, most of the individuals have five fingers instead of having six.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.