Which statement about polydactyly is true regarding expressivity?

Polydactyly, a condition characterized by the presence of extra fingers or toes, demonstrates variable expressivity, meaning that the severity and manifestation of the trait can differ significantly among individuals. This variability can include differences in the number of extra digits, their size, and functionality, even among genetically similar individuals.

This phenomenon can be attributed to a variety of factors, including environmental influences and genetic background, which can interact with the underlying genetic predisposition. As a condition that features this variability in expression, polydactyly serves as an excellent example of how a single genetic trait can lead to a range of physical presentations within a population.

In contrast, other options do not accurately represent the nature of expressivity in polydactyly. For instance, saying it is "always expressed in the same manner" contradicts the concept of expressivity, as it implies uniformity that does not hold true. The relationship with penetrance is also more nuanced than simply stating it has no relation; penetrance and expressivity are interconnected aspects of the genetics of traits. Finally, defining polydactyly solely as a recessive trait overlooks the complexities of genetic inheritance since it can also be inherited in a dominant manner.