Autosomal dominant inheritance is due to a dominant allele carried on one of the autosomes. Autosomal dominant alleles need only be inherited from one parent, either the mother or the father, in order to be expressed in the phenotype. Because of this, any child has a 50 percent chance of inheriting the allele and expressing the trait if one parent has it.
Many normal human traits are due to autosomal dominant alleles, including the presence of dimples, a cleft chin, and a widow's-peak hairline. Note that dominant does not necessarily mean common. Dominant alleles can be rare in a population, and do not spread simply because they are dominant. This phenomenon is explained by the theory known as Hardy-Weinberg equilibrium.
There are hundreds of medical conditions due to autosomal dominant alleles, most of them very rare. They include neurodegenerative disorders such as Huntington's disease, a variety of deafness syndromes, and metabolic disorders such as familial hypercholesterolemia (affecting blood cholesterol levels) and variegate porphyria (affecting the oxygen-carrying porphyrin molecule). Table 1 lists some other examples.
Because inheritance of a harmful dominant allele can be lethal, these alleles tend to be quite rare in the population, and new mutations account mutations changes in for many cases of these conditions. Exceptions include late-onset disorders such as Huntington's disease, in which parents may pass on the gene to offspring before developing the symptoms of the disease. Other exceptions arise from incomplete penetrance, in which the allele is present, but (for reasons usually unknown) it is not expressed. Genomic imprinting (see below) may explain some cases of incomplete penetrance. Variable expressivity is also possible, in which different individuals express the trait with different levels of severity.
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