Final answer:
Phenotypes in threshold traits are determined by a combination of genetic and environmental factors, as well as the complex interplay of multiple genes. Environmental influences like sun exposure and incubation temperature can modify traits, while polygenic inheritance and natural selection impact population variation and trait distribution.
Step-by-step explanation:
The final phenotype in threshold traits is influenced not only by genetics but also by environmental factors and interactions among multiple genes. Environmental factors like sun exposure can affect skin color, an observable phenotype, with prolonged exposure leading to darker skin. In certain species, such as turtles, environmental conditions like incubation temperature can even determine sex, a process known as temperature-dependent sex determination (TSD).
In addition to the environment, natural selection plays a role in phenotype development, but it can be limited by polymorphic relationships. For example, beach-dwelling mice with different coat colors may experience different survival odds depending on their ability to camouflage. A light-colored mouse on a sandy beach has a high survival rate, just as a dark-colored mouse does in grassy patches, but mice with intermediate colors are more visible to predators and have lower survival rates. Thus, natural selection may maintain distinct phenotypes rather than favoring a transition to a more beneficial trait that would involve an intermediate, less beneficial state.
Polygenic inheritance is another aspect affecting the phenotype, where multiple genes contribute to a characteristic, such as skin color or height. These genes may have complimentary or synergistic effects and are often expressed in a sequential or simultaneous manner that influences phenotype complexity. Polygenic traits usually result in a bell-shaped curve distribution of these traits within a population.