Some traits appear even when only one copy of the responsible allele is present. Autosomal dominant inheritance occurs when a dominant allele on a non-sex chromosome produces the phenotype. Heterozygous individuals show the trait; homozygous dominant individuals may be more severely affected or, in some cases, nonviable.

Huntington’s disease and achondroplasia follow this pattern. Each child of an affected parent has a 50 percent chance of inheriting the allele and expressing the condition. The trait typically appears in every generation, though incomplete penetrance can occasionally skip a generation.

Genetic counseling relies on accurate family histories and molecular testing to confirm the causative variant. Because the allele is expressed in heterozygotes, dominant disorders often present earlier and more visibly than recessive ones. New mutations also account for a portion of cases.

• Trait expressed with one dominant allele
• Gene located on an autosome
• Transmitted to 50 percent of offspring on average
• Usually appears in successive generations