Practice question

Fully penetrant autosomal dominant allele manifests phenotype in heterozygote, so individual cannot express trait without receiving mutant allele from at least one parent. Rare de novo mutations can create isolated affected individuals, but most affected persons inherit allele from one affected parent, creating vertical transmission where trait appears in each generation continuously. Normal homozygotes never produce affected child unless other parent contributes dominant allele. This inheritance contrasts sharply with recessive where affected individuals can have unaffected parents. Thus presence of affected parent in most cases supports dominant mode.