Incomplete penetrance

Referring to the presence of a gene that is not phenotypically expressed in all members of a family with the gene.

A study of generally uniform line of Drosophila flies shows that when moisture content is high during development, more flies in the population develop abnormal abdominal phenotype. Therefore greater percent of penetrance is observed with increased moisture. The reverse is the case if the moisture content is low.

Penetrance is the frequency of phenotypic expression of a gene or allele in a family.

Yes, Huntington's disease is autosomal dominant with high penetrance. Every individual who inherits the allele gets the disease.