This document discusses how a genetic marker can identify a mutant allele for sickle cell anemia even if the causative mutation does not fall within the cut site of the marker. The sickle cell mutation serves as both the cause of the disease and a genetic marker because it is located within the cut site of the DdeI enzyme used to identify it. Genetic markers can identify mutant alleles through genetic linkage, as the marker and mutation tend to be inherited together due to their proximity on the chromosome.