The claim that having six fingers is a dominant trait is true for certain types of polydactyly. Polydactyly is the condition of having more than the typical number of fingers or toes. While having five fingers is common, the genetic basis for polydactyly often shows a dominant inheritance pattern. This means that only one copy of the gene responsible for extra digits is needed to express the trait.
Understanding Dominant Genes
In genetics, a dominant trait manifests even if only one copy of the responsible gene is present. Conversely, recessive traits require two copies of the gene to be expressed. The gene responsible for polydactyly is, in many cases, dominant, so even a single copy of that gene in an individual can lead to the development of extra digits.
Why Don't More People Have Six Fingers?
While the gene for extra digits is dominant in certain forms of polydactyly, it doesn't mean most people have six fingers. Several factors contribute to this:
- Rarity of the Gene: The specific gene causing this type of polydactyly isn't prevalent in the human population.
- Incomplete Penetrance: Even with the dominant gene present, it doesn't always result in an extra digit in every individual carrying it. This is known as incomplete penetrance. The gene's expression can be influenced by other genetic factors.
- Negative Selection: Historically, individuals with polydactyly may have faced challenges impacting their survival and reproduction. Natural selection would reduce the frequency of the gene over time.
- Genetic Heterogeneity: Not all cases of polydactyly are caused by the same dominant gene. Some cases are caused by recessive genes, others are due to different genetic mutations.
Examples and Studies
Many studies support the idea of a dominant gene for polydactyly. Research on fruit flies has shown that mutations can lead to extra limbs. Furthermore, studies of human families with polydactyly often show a dominant inheritance pattern. The medical literature extensively documents autosomal dominant postaxial polydactyly (PAP-A) which is a specific type exhibiting this inheritance pattern. The biblical reference to Goliath's family having extra fingers and toes also suggests a possible hereditary condition, possibly fitting this dominant trait.
