Final answer:
An individual expresses an autosomal dominant disease when they inherit at least one dominant allele responsible for the disease. The disease can be expressed even if the second allele is normal, and carriers of such a dominant gene have a significant chance of passing it to their offspring.
Step-by-step explanation:
An autosomal dominant disease occurs when an individual has at least one dominant allele for the gene associated with the disease. This means that even a single copy of the faulty gene can lead to the manifestation of the disease.
Within autosomal dominant inheritance, diseases like Huntington's disease, achondroplastic dwarfism, and Marfan syndrome can occur if a person inherits a dominant mutant allele from one of their parents.
Even if the other allele is normal, the dominant nature of the disease-causing allele ensures that the individual will express the disorder. This differs from autosomal recessive disorders, where two copies of the recessive allele are needed for the disorder to be apparent.
For someone who carries an autosomal dominant allele, there's a 50 percent chance of passing this allele to each offspring if the other parent does not carry the allele at all (is homozygous recessive). If the person with the disorder is homozygous for the dominant allele, their offspring will always inherit the disorder, assuming the other parent is unaffected.