Incomplete penetrance can refer to autosomal recessive and dominant modes of inheritance. Most diseases have complete penetrance. That is the individual with the gene mutation shows the physical signs/symptoms of the disease. Cases where the individuals have been confirmed to have the mutant gene responsible but show no sign of any symptoms are known as in complete penetrance. The disease may appear to skip generations and appear in later progeny. In fact the disease hasn't been 'missing' is simply hasn't expressed itself. Sometimes in diseases such as exercise induced collapse an external/environmental factor may be required to 'trigger' the disease to express itself.