Karyotype — Robertsonian Translocation 14;21 (Inherited Down Carrier)

A 28-year-old primigravida presents for genetic counseling after her first child was born with Down syndrome. Karyotype of the child shows 46 chromosomes with three copies of chromosome 21 material. Parental karyotypes are performed; the mother is found to have 45 chromosomes. The structure marked **A** in the diagram represents the Robertsonian fusion chromosome der(14;21) present in the mother's karyotype. Which of the following best explains why this carrier mother has a significantly LOWER empirical recurrence risk (~10-15%) compared to the theoretical risk of 33% for subsequent pregnancies?