[citation needed] These disorders are often best known in some classic form, such as the homozygous recessive case of a particular mutation that is widespread in some population.
Although compound heterozygosity as a cause of genetic disease had been suspected much earlier, widespread confirmation of the phenomenon was not feasible until the 1980s, when polymerase chain reaction techniques for amplification of DNA made it cost-effective to sequence genes and identify polymorphic alleles.
[citation needed] For example, hemochromatosis is the name given to several different heritable diseases with the same outcome, excess absorption of iron.
[1] But at each gene locus associated with the disease, there is the possibility of compound heterozygosity, often caused by inheritance of two unrelated alleles, of which one is a common or classic mutation, while the other is a rare or even novel one.
In the case of hemochromatosis, penetrance is incomplete, even for the classic HFE mutation, and is affected by gender, diet, and behaviors such as alcohol consumption.
However, the three were discovered and named separately, and each represents a distinct molecular point of failure in a subunit that is required for activation of the enzyme.