Ataxia-Telangiectasia (AR)

A 7-year-old girl is referred for progressive gait instability since age 18 months. On examination, she has cerebellar ataxia with truncal titubation, oculomotor apraxia with head thrusts to initiate saccades, and telangiectasias over the bulbar conjunctivae and nasal bridge. Laboratory studies show low serum IgA, low IgG2, and markedly elevated alpha-fetoprotein. Her parents are first cousins with no family history of similar disease in previous generations. Her older brother was similarly affected and died of T-cell leukemia at age 12. The pedigree pattern shown in the diagram at **B** is most consistent with which inheritance pattern?