Results of the southern blot assay at the FMR-1 locus.

Lane numbers correspond to individuals in the pedigree and fragment sizes are indicated in kilobases at the side of the figure. The father (I.1) exhibits a normal pattern with a unique 2.8-kb band corresponding to his unmethylated active X-chromosome, whereas in normal females both bands of 2.8 kb and 5.2 kb are present and correspond to their active and inactive alleles (individual II.1). The mother (I.2) exhibits a pattern of 4 bands, suggestive of a premutated allele in the methylated and unmethylated states (bands of ∼5.5 kb and 3 kb, respectively). Subjects I.2 and II.1 exhibit a random X-chromosome inactivation as indicated by an equal intensity of each fragment. The twins (II.2 and II.3) show a pattern of 4 bands like their mother but 2 bands are weaker (white arrowheads), indicating a skewed X-chromosome inactivation.