Familial cortical myoclonic tremor with epilepsy type 1 (FCMTE1) is caused by (TTTTA)exp(TTTCA)exp repeat expansions in SAMD12, while pure (TTTTA)exp is polymorphic. Our investigation focused on the origin and evolution of pure (TTTTA)exp and (TTTTA)exp(TTTCA)exp at this locus. We observed a founder effect between them. The phylogenetic analysis suggested that the (TTTTA)exp(TTTCA)exp might be generated from pure (TTTTA)exp through infrequent transformation events. Long-read sequencing revealed somatic generation of (TTTTA)exp(TTTCA)exp from pure (TTTTA)exp, likely via long segment (TTTCA) repeats insertion. Our findings indicate close relationships between the non-pathogenic (TTTTA)exp and the pathogenic (TTTTA)exp(TTTCA)exp, with dynamic interconversions. This sheds light on the genesis of pathogenic repeat expansions from ancestral premutation alleles. Our results may guide future studies in detecting novel repeat expansion disorders and elucidating repeat expansion mutational processes, thereby enhancing our understanding of human genomic variation.