<unk>  is one of the causes of variation in genetic disease. The diagnosis and nomenclature for such disorders sometimes reflects history, because most diseases were first observed and classified based on biochemistry and pathophysiology before genetic diagnosis was available. Some genetic disorders are really a family of related disorders that occur in the same metabolic pathway, or in related pathways. Naming conventions for the disease became established before precise molecular diagnosis was possible. For example, hemochromatosis is the name given to several different heritable diseases with the same outcome, excess absorption of iron. These variants all reflect a failure in a metabolic pathway associated with iron metabolism, however mutations that cause hemochromatosis can occur at different gene loci. Mutations have occurred at each locus many times, and a few such mutations have become widespread in some population. The fact that multiple loci are involved is the primary cause for the variant forms of hemochromatosis and its outcome. This variation is caused not by<unk> , but rather by the fact that several different enzyme defects can cause the disease. Clinically, most cases of hemochromatosis are found in homozygotes for the most common mutation in the HFE gene. But at each gene locus associated with the disease, there is the possibility of<unk> , 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. For some genetic diseases, environmental cofactors are an important determinant of variation and outcome. 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.<unk>  heterozygotes are often observed only through subclinical symptoms such as excess iron. Disease is rarely observed in such<unk>  heterozygotes unless other causal factors (such as alcoholism) are present. As a result,<unk>  for hemochromatosis may be more common than diagnosis based on pathology would suggest. Some genetic diseases are named more precisely, and represent a single point of failure on a metabolic pathway. For example, Tay–Sachs disease, GM2-gangliosidosis, AB variant, and Sandhoff disease might easily have been defined together as a single disease, because the three disorders are associated with failure of the same enzyme and have the same outcome. 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. For all three disorders,<unk>  is responsible for variant forms. For example, both TSD and Sandhoff disease have a more common infantile form and several late-onset variants. Post-infantile forms, which are rare, are generally caused by the inheritance of two unrelated alleles, of which one is usually a classic mutation, while the other is a rare or even novel one.
Compound heterozygosity
Allelic heterogeneity
Heteroplasmy
Uniparental disomy
Anticipation 
Expressivity 
Trisomy
Genomic imprinting