autosomal dominant
Imagine a light switch that turns on if even one of two wires carries current. In autosomal dominant inheritance, a single working copy of a particular allele is enough to produce the trait, even though every person carries two copies of each non-sex gene — one from each parent.
The gene sits on an autosome (a numbered chromosome 1–22, not the X or Y), so males and females are affected with roughly equal frequency. An affected parent who carries one dominant and one ordinary allele passes the dominant allele to each child with a 50% chance, so the trait typically appears in every generation of a family — a vertical pattern on a pedigree.
Because only one altered copy is needed, dominant conditions are often visible even when rare. But the picture can be softened: some carriers never show the trait (reduced penetrance) and others show it mildly or severely (variable expressivity). A dominant condition can also arise fresh in a child through a new mutation, with no affected parent at all.
Huntington disease is autosomal dominant: a single expanded copy of the HTT gene is enough to cause it, and a parent passes it to each child with a 50% chance.
One dominant allele, a 50% transmission risk, and a trait seen in every generation.
Affected children of an affected parent are common, but absence of family history does not rule out a dominant condition — new mutations and reduced penetrance both break the expected pattern.