A 35-year-old man presents to the genetics clinic with a family history of Huntington disease. His father was diagnosed at age 40 and died at age 65 with progressive chorea and dementia. The patient's paternal grandfather was unaffected. The pedigree shows the feature marked **A** — vertical transmission across generations with disease appearing in every generation without skipped generations. Which of the following best explains why this pattern is characteristic of autosomal dominant inheritance?