Dominant diseases are single gene disorders that occur in the heterozygous state - when an individual possesses one defective copy of the relevant gene but also one normal copy.
Somehow, the defective copy (which is known as a mutant allele or disease allele) is able to overcome the effect of a normal copy.
Most people with dominant diseases are heterozygous because two affected people would have to marry in order to produce homozygous children - children with a double dose of the mutant allele. In rare cases where this happens, the disease symptoms in homozygous people are generally more severe.
Because everyone carrying a dominant mutant allele generally shows the disease symptoms, dominant diseases can be traced through family pedigrees and appear to spread vertically (see figure).
This is completely different to the inheritance patterns seen in recessive diseases , which tend to skip generations. It may be possible to trace a dominant disease through many generations.
One interesting example is variegate porphyria, a disease caused by the loss of one copy of the PPOX gene on chromosome 1 resulting in an inability to synthesise haem (an essential component of haemoglobin). In South Africa, nearly all cases of this disease can be traced back to one person living in the late seventeenth century. This person was the source of the mutation that originally generated the disease allele.
Other examples of dominant diseases include Huntington's disease and myotonic dystrophy.