Figure 4: Methylation analysis. PCR amplification of the trinucleotide CAG repeat before and after digestion with HhaI and HpaII nucleases. Selective amplification of the paternal allele before digestion is noted and should be attributed to the double dosage of Xq due to the presence of the isochromosome. Absence of the maternally originated allele after digestion indicates that this allele was digested because of its unmethylated active status. F: Father C: Child M: Mother.