We can detect the SNP in the S allele because it destroys a DdeI restriction site, leading to DNA ...

The DdeI target sequence is 5 - CTNAG - 3, where N indicates that any of the four nucleotides (A, T, C, or G) can occur in the middle of the 5-bp sequence as long as the variable nucleotide is flanked by CT and AG dinucleotide combinations. Thus, you would be unable to detect the difference between CCTGAG and CCTTAG by restriction fragment analysis. Several other techniques may help detect this particular mutation:
(1 ) You could design a molecular probe specific for the mutant sequence, although it may be difficult to detect a one-base-pair change in sequence. The probe would have to be very short, and the annealing conditions would need to be fairly stringent so that the probe would not anneal based solely on the homologous regions around the mutation.
(2 ) You could also look for changes at the protein level. This mutation occurred in an exon region, so you might expect to see a change in the amino acid sequence (depending on the translation frame). For instance, if GAG is one codon and TAG is the SNP's codon, you would see a Glu  Stop mutation and could look for the presence of a truncated protein in the mutant allele.
(3 ) Alternatively, it may be easiest to design a short primer upstream of this mutation and sequence the region directly using genomic DNA as a template for PCR (a technique that is described in detail in later chapters).