Final answer:
The gene encoding thermogenin is indeed expressed only in the brown adipose tissue of mammals, which is prevalent in newborns and helps in nonshivering thermogenesis, important for heat generation.
Step-by-step explanation:
The statement that a gene encoding the protein thermogenin is expressed only in the brown adipose tissue of mammals is true. Thermogenin, also known as uncoupling protein 1 (UCP1), is indeed a protein that is predominantly found in brown adipose tissue. This specialized tissue is capable of nonshivering thermogenesis, which is the process of heat production in organisms. It is packed with mitochondria rich in iron-containing cytochromes, giving it a brown appearance. The role of thermogenin is to uncouple oxidative phosphorylation, thereby allowing the mitochondria to generate heat instead of ATP, which is crucial for maintaining body temperature, particularly in infants and animals adapted to cold environments.
Newborn humans have deposits of brown fat that contain thermogenin, which helps to heat the blood circulating in their bodies. This is important as newborns can lose heat quickly due to their large surface area to mass ratio. In adults, the presence of brown fat, and consequently the expression of thermogenin, may be limited but can be found in those living in colder climates, where it serves to facilitate nonshivering thermogenesis.