Final answer:
IgE is the correct answer, as it is associated with a constant region gene that includes a non-functional pseudogene, essential in understanding the antibody responses.
Step-by-step explanation:
The correct answer to the question of which Constant region gene has a non-functional pseudogene is IgE, denoted by the letter 'E'. Based on the reference information provided, we see that IgE is a class of antibody that has a specific connotation within the adaptive immune system, primarily related to anti-parasitic defenses and allergic responses. The mention of IgE’s role in mast cell degranulation during an allergic response implies that it possesses a unique Fc region that allows for this activity. In immunology, functional genes encode for parts of the immune system, while pseudogenes are sequences that resemble genes but are non-functional. Therefore, understanding that the IgE's constant region gene includes a non-functional pseudogene is relevant for understanding the diversity and regulation of antibody responses in the human immune system.