Bloom Syndrome (AR)

A 6-year-old boy of Ashkenazi Jewish descent presents with severe growth retardation (height and weight <3rd percentile), a long narrow face with prominent nose, and a sun-sensitive erythematous telangiectatic rash in a butterfly distribution over the malar regions. He has recurrent upper respiratory infections and café-au-lait spots. His pedigree shows an affected younger sister, unaffected non-consanguineous parents, and no affected ancestors. Cytogenetic analysis reveals a dramatically increased rate of sister chromatid exchanges (approximately 10-fold higher than normal) and quadriradial chromosome figures. The inheritance pattern shown in the pedigree (marked **A** in the diagram) is consistent with which of the following genetic mechanisms?