Autosomal Dominant Inheritance

Autosomal dominant inheritance is marked by the primary feature that one copy of an allele is sufficient for expression of a trait; the gene located on one of the 22 autosomes (that is, not the X or Y chromosome) is expressed in the heterozygous state.

Each affected person has at least one affected parent. Exceptions may occur for one of three reasons:

The affected person is the result of a new mutation.
The parent transmitting the gene did not show the trait, even though he or she carried the allele; this is known as incomplete penetrance, that is, the inconsistent phenotypic expression of a gene even though the gene is present.
The parent of the affected individual expressed the gene but in ways that were not readily recognized; this is known as variable expressivity.

Other characteristics of autosomal dominance:

An affected person has a 50% chance of passing the trait to a child.
Males and females are equally likely to be affected.
Two affected people can have an unaffected child.

In addition to the basic interpretation of an autosomal dominant pedigree, Bayesian analysis can refine risk assessment by taking into account the full family and clinical information.