A 3-year-old developmentally delayed girl presents to the pediatric neurologist for evaluation of new onset seizures. The parents are also concerned because the child frequently exhibits inappropriate outbursts of laughter. Physical examination is significant for abnormal facies marked by microcephaly, deep-set eyes, and a large mouth with a protruding tongue. The child’s gait is unstable. The most likely diagnosis is an example of which of the following genetic phenomena?
(A) Anticipation
(B) Heteroplasmy
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(D) Locus heterozygosity
(E) Mosaicism