Final answer:
The gene for Retinitis pigmentosa appears on the X chromosome in some forms of the disorder, which explains why males are more commonly affected by X-linked forms of RP.
Step-by-step explanation:
Retinitis pigmentosa (RP) is a group of eye disorders that affect the retina. Your question asks for clarification on the nature of the genetic mutation that causes RP, specifically whether it only occurs in males, and which chromosome the gene for it appears on. The correct answer in this case is b. The gene for it appears on the X chromosome. RP is a heterogeneous disorder, meaning that it can be caused by mutations in various genes located on different chromosomes, including autosomal (non-sex) chromosomes and the X chromosome.
However, your statement that RP only occurs in males suggests a reference to an X-linked form of RP, where the defective gene is located on the X chromosome. In X-linked recessive disorders like some forms of RP, males are more commonly affected because they have a single X chromosome. Females have two X chromosomes, so a defective gene on one of their X chromosomes would typically need to be matched with a defective gene on the other X chromosome for the disorder to manifest, which is less common.