(A) Example of a CAG short tandem repeat with genotype 4/6. (B) Slip mispairing: During DNA replication, nascent DNA copy (in purple) slips back one tandem repeat, resulting in duplicate replication of tandem repeat #5. Results in additional copy (total 7) on new strand. (C) Pedigree of Myotonic Dystrophy in three generations with anticipation. Grandmother has 75 tandem repeats at the DMPK gene, no symptoms. Mother has expansion of 530 repeats, with mild symptoms (no respiratory distress) at age 40. Grandson has progressive expansion to 1200 repeats, manifesting with myotonia and respiratory failure at one year of age.
