Both the assertion and reason are true, and the reason correctly explains the assertion. According to the context, RFLP involves observing 'changes in length of DNA fragments' when DNA samples are digested with restriction enzymes. The context further explains that physical genome maps can be compared by 'noticing the gain or loss of certain restriction sites resulting in observed RFLP.' Figure 5.3 illustrates this concept clearly: genome (1) has restriction site 'c' present, resulting in two smaller fragments, while genome (2) lacks this site, producing one larger fragment. This demonstrates that different individuals have different positions of restriction sites (some sites may be present or absent), which directly causes the variation in fragment lengths when the same restriction endonuclease is used. Thus, the reason correctly explains why RFLP occurs.