Final answer:
The question's reference to gain-of-function disorders relating specifically to the buttocks is a misconception. These disorders pertain to genetic mutations that lead to enhanced protein functions, and are not related to cultural physical preferences. Prader-Willi syndrome is an example of a genetic disorder affecting multiple body systems due to loss of function of certain genes.
Step-by-step explanation:
The question seems to be mixing up cultural preferences regarding physical characteristics with genetic disorders, which are two distinct concepts. The preference for certain physical traits like large buttocks in West Africa is a cultural and evolutionary topic rather than a medical disorder. Gain-of-function disorders refer to genetic conditions where a mutation causes a new or enhanced function of a protein rather than a cultural preference for a physical trait.
An example of a gain-of-function disorder is not specified to any part of the body, such as the buttocks, but affects various biological functions depending on the gene involved. For instance, Prader-Willi syndrome is a genetic disorder that involves loss of function, rather than gain, of specific genes on chromosome 15 and affects many parts of the body, leading to issues like constant hunger and potential obesity, not a preference for physical features.