It is most common in certain European populations (such as the Irish and Norwegians) and occurs in 0.6% of the population. Men with the disease are 24 times more likely to experience symptoms than affected women.

Affected individuals over age 40 or who have high serum ferritin levels are at risk for developing cirrhosis. Iron overload increases the risk of hepatocellular carcinoma. This risk is greater in those with cirrhosis but is still present in those without cirrhosis. Significant problems occur in around one in ten.

Juvenile hemochromatosis (or hemochromatosis type 2) is, as its name indicates, a form of hemochromatosis which emerges during youth.

There are two forms:

- "HFE2A" is associated with hemojuvelin

- "HFE2B" is associated with hepcidin antimicrobial peptide

Some sources only specifically include hemojuvelin as a cause of juvenile hemochromatosis.

Pyruvate kinase deficiency happens worldwide, however northern Europe, and Japan have many cases. The prevalence of pyruvate kinase deficiency is around 51 cases per million in the population (via gene frequency).

The causes of neonatal hemochromatosis are still unknown, but recent research has led to the hypothesis that it is an alloimmune disease. Evidence supporting this hypothesis includes the high rate among siblings (>80%). This evidence along with other research indicates that neonatal hemochromatosis could be classified as a congenital alloimmune hepatitis.

The condition is sometimes confused with juvenile hemochromatosis, which is a hereditary hemochromatosis caused by mutations of a gene called hemojuvelin. While the symptoms and outcomes for these two diseases are similar, the causes appear to be different.

Genetic disorders may also be complex, multifactorial, or polygenic, meaning they are likely associated with the effects of multiple genes in combination with lifestyles and environmental factors. Multifactorial disorders include heart disease and diabetes. Although complex disorders often cluster in families, they do not have a clear-cut pattern of inheritance. This makes it difficult to determine a person’s risk of inheriting or passing on these disorders. Complex disorders are also difficult to study and treat, because the specific factors that cause most of these disorders have not yet been identified. Studies which aim to identify the cause of complex disorders can use several methodological approaches to determine genotype-phenotype associations. One method, the genotype-first approach, starts by identifying genetic variants within patients and then determining the associated clinical manifestations. This is opposed to the more traditional phenotype-first approach, and may identify causal factors that have previously been obscured by clinical heterogeneity, penetrance, and expressivity.

On a pedigree, polygenic diseases do tend to "run in families", but the inheritance does not fit simple patterns as with Mendelian diseases. But this does not mean that the genes cannot eventually be located and studied. There is also a strong environmental component to many of them (e.g., blood pressure).

- asthma

- autoimmune diseases such as multiple sclerosis

- cancers

- ciliopathies

- cleft palate

- diabetes

- heart disease

- hypertension

- inflammatory bowel disease

- intellectual disability

- mood disorder

- obesity

- refractive error

- infertility

Haematologists have identified a number of variants. These can be classified as below.

- Overhydrated hereditary stomatocytosis

- Dehydrated HSt (hereditary xerocytosis; hereditary hyperphosphatidylcholine haemolytic anaemia)

- Dehydrated with perinatal ascites

- Cryohydrocytosis

- 'Blackburn' variant.

- Familial pseudohyperkalaemia

There are other families that do not fall neatly into any of these classifications.

Stomatocytosis is also found as a hereditary disease in Alaskan malamute and miniature schnauzer dogs.

The cause for these hereditary conditions is now understood to be various mutations in the erythrocyte membrane protein, band 3. It is this protein which mediates the cation leaks which are characteristic of this disease.

Pyruvate kinase deficiency is an inherited metabolic disorder of the enzyme pyruvate kinase which affects the survival of red blood cells. Both autosomal dominant and recessive inheritance have been observed with the disorder; classically, and more commonly, the inheritance is autosomal recessive. Pyruvate kinase deficiency is the second most common cause of enzyme-deficient hemolytic anemia, following G6PD deficiency.

This disease is endemic in Portuguese locations Póvoa de Varzim and Vila do Conde (Caxinas), with more than 1000 affected people, coming from about 500 families, where 70% of the people develop the illness. ll the analysed Portuguese families presented the same haplotype (haplotype I) associated with the Met 30 mutation. In northern Sweden, more specifically Piteå, Skellefteå and Umeå, 1.5% of the population has the mutated gene. There are many other populations in the world who exhibit the illness after having developed it independently.

A genetic disorder is a genetic problem caused by one or more abnormalities in the genome, especially a condition that is present from birth (congenital). Most genetic disorders are quite rare and affect one person in every several thousands or millions.

Genetic disorders may be hereditary, passed down from the parents' genes. In other genetic disorders, defects may be caused by new mutations or changes to the DNA. In such cases, the defect will only be passed down if it occurs in the germ line. The same disease, such as some forms of cancer, may be caused by an inherited genetic condition in some people, by new mutations in other people, and mainly by environmental causes in other people. Whether, when and to what extent a person with the genetic defect or abnormality will actually suffer from the disease is almost always affected by the environmental factors and events in the person's development.

Some types of recessive gene disorders confer an advantage in certain environments when only one copy of the gene is present.

Familial amyloid polyneuropathy (FAP), also called transthyretin-related hereditary amyloidosis, transthyretin amyloidosis abbreviated also as ATTR (hereditary form), or Corino de Andrade's disease, is an autosomal dominant neurodegenerative disease. It is a form of amyloidosis, and was first identified and described by Portuguese neurologist Mário Corino da Costa Andrade, in 1952. FAP is distinct from senile systemic amyloidosis (SSA), which is not inherited, and which was determined to be the primary cause of death for 70% of supercentenarians who have been autopsied.

FAP can be ameliorated by liver transplantation.

Those with hereditary elliptocytosis have a good prognosis, only those with very severe disease have a shortened life expectancy.

Hereditary spherocytosis is the most common disorder of the red cell membrane and affects 1 in 2,000 people of Northern European ancestry. According to Harrison's Principles of Internal Medicine, the frequency is at least 1 in 5,000.

May–Hegglin anomaly (MHA), also known as Döhle leukocyte inclusions with giant platelets and macrothrombocytopenia with leukocyte inclusions, is a rare genetic disorder of the blood platelets that causes them to be abnormally large.

HSAN I constitutes a clinically and genetically heterogeneous group of diseases of low prevalence. Detailed epidemiological data are currently not available. The frequency of the disease is still reflected by reports of a handful affected families. Although the impressive clinical features of HSAN I are seen by neurologists, general practitioners, orthopedists, and dermatologists, the condition might still be under-recognized particularly for sporadic cases and patients who do not exhibit the characteristic clinical features.

The exact mechanisms of these diseases are not well understood. GNE/MNK a key enzyme in the sialic acid biosynthetic pathway, and loss-of-function mutations in GNE/MNK may lead to a lack of sialic acid, which in turn could affect sialoglycoproteins. GNE knockout mice show problems similar to people with IBM and in people with IBM dystroglycan has been found to lack sialic acid. However, the part of the dystroglycan that is important in muscle function does not seem to be affected. Another protein, neural cell adhesion molecule is under-sialyated in people with IBM, but as of 2016 it had no known role in muscle function.

Experimental gene therapy exists to treat hereditary spherocytosis in lab mice; however, this treatment has not yet been tried on humans due to all of the risks involved in human gene therapy.

Hereditary gelsolin amyloidosis is a cutaneous condition inherited in an autosomal dominant fashion.

The condition was first described in 1969, by the Finnish ophthalmologist Jouko Meretoja, and is also known as Familial amyloid neuropathy type IV, Meretoja syndrome, Hereditary amyloidosis, Finnish type.

The disorder primarily associated with eye, skin and cranial nerve symptoms. It is a form of amyloidosis, where the amyloid complexes are formed from fragments of the protein gelsolin in the plasma, due to a mutation in the GSN gene (c.654G>A or c.654G>T).

The incidence of hereditary elliptocytosis is hard to determine, as many sufferers of the milder forms of the disorder are asymptomatic and their condition never comes to medical attention. Around 90% of those with this disorder are thought to fall into the asymptomatic population. It is estimated that its incidence is between 3 and 5 per 10,000 in the United States, and that those of African and Mediterranean descent are of higher risk. Because it can confer resistance to malaria, some subtypes of hereditary elliptocytosis are significantly more prevalent in regions where malaria is endemic. For example, in equatorial Africa its incidence is estimated at 60-160 per 10,000, and in Malayan natives its incidence is 1500-2000 per 10,000. Almost all forms of hereditary elliptocytosis are autosomal dominant, and both sexes are therefore at equal risk of having the condition. The most important exception to this rule of autosomal dominance is for a subtype of hereditary elliptocytosis called hereditary pyropoikilocytosis (HPP), which is autosomal recessive.

There are three major forms of hereditary elliptocytosis: common hereditary elliptocytosis, spherocytic elliptocytosis and southeast Asian ovalocytosis.

Common hereditary elliptocytosis is the most common form of elliptocytosis, and the form most extensively researched. Even when looking only at this form of elliptocytosis, there is a high degree of variability in the clinical severity of its subtypes. A clinically significant haemolytic anaemia occurs only in 5-10% of sufferers, with a strong bias towards those with more severe subtypes of the disorder.

Southeast Asian ovalocytosis and spherocytic elliptocytosis are less common subtypes predominantly affecting those of south-east Asian and European ethnic groups, respectively.

The following categorisation of the disorder demonstrates its heterogeneity:

- Common hereditary elliptocytosis (in approximate order from least severe to most severe)

- With asymptomatic carrier status - "individuals have no symptoms of disease and diagnosis is only able to be made on blood film"

- With mild disease - "individuals have no symptoms, with a mild and compensated haemolytic anaemia"

- With sporadic haemolysis - "individuals are at risk of haemolysis in the presence of particular comorbidities, including infections, and vitamin B deficiency"

- With neonatal poikilocytosis - "individuals have a symptomatic haemolytic anaemia with poikilocytosis that resolves in the first year of life"

- With chronic haemolysis - " individual has a moderate to severe symptomatic haemolytic anaemia (this subtype has variable penetrance in some pedigrees)"

- With homozygosity or compound heterozygosity - "depending on the exact mutations involved, individuals may lie anywhere in the spectrum between having a mild haemolytic anaemia and having a life-threatening haemolytic anaemia with symptoms mimicking those of HPP (see below)"

- With pyropoikilocytosis (HPP) - "individuals are typically of African descent and have a life-threateningly severe haemolytic anaemia with micropoikilocytosis (small and misshapen erythrocytes) that is compounded by a marked instability of erythrocytes in even mildly elevated temperatures (pyropoikilocytosis is often found in burns victims and is the term is commonly used in reference to such people)

- South-east Asian ovalocytosis (SAO) (also called stomatocytic elliptocytosis) - "individuals are of South-East Asian descent (typically Malaysian, Indonesian, Melanesian, New Guinean or Filipino, have a mild haemolytic anaemia, and has increased resistance to malaria"

- Spherocytic elliptocytosis (also called hereditary haemolytic ovalocytosis) - "individuals are of European descent and elliptocytes and spherocytes are simultaneously present in their blood"

This disease is more common in women and an association with the gene FLT4 has been described. FLT4 codes for VEGFR-3, which is implicated in development of the lymphatic system.

Milroy's disease is also known as primary or hereditary lymphedema type 1A or early onset lymphedema.

It is a very rare disease with only about 200 cases reported in the medical literature. Milroy's disease is an autosomal dominant condition caused by a mutation in the FLT4 gene which encodes of the vascular endothelial growth factor receptor 3 (VEGFR-3) gene located on the long arm (q) on chromosome 5 (5q35.3).

In contrast to Milroy's disease (early onset lymphedema type 1A,) which typically has its onset of swelling and edema at birth or during early infancy, hereditary lymphedema type II, known as Meige disease, has its onset around the time of puberty. Meige disease is also an autosomal dominant disease. It has been linked to a mutations in the ‘forkhead’ family transcription factor (FOXC2) gene located on the long arm of chromosome 16 (16q24.3). About 2000 cases have been identified. A third type of hereditary lymphedema, that has an onset after the age of 35 is known as lymph-edema tarda.

Because lack of sialic acid appears to be part of the pathology of IBM caused by GNE mutations, clinical trials with sialic acid supplements, and with a precursor of sialic acid, N-Acetylmannosamine, have been conducted, and as of 2016 further trials were planned.

MHA is believed to be associated with the "MYH9" gene. The pathogenesis of the disorder had been unknown until recently, when autosomal dominant mutations in the gene encoding non-muscle myosin heavy chain IIA ("MYH9") were identified. Unique cytoplasmic inclusion bodies are aggregates of nonmuscle myosin heavy chain IIA, and are only present in granulocytes. It is not yet known why inclusion bodies are not present in platelets, monocytes, and lymphocytes, or how giant platelets are formed. "MYH9" is also found to be responsible for several related disorders with macrothrombocytopenia and leukocyte inclusions, including Sebastian, Fechtner, and Epstein syndromes, which feature deafness, nephritis, and/or cataract. MHA is also a feature of the Alport syndrome (hereditary nephritis with sensorineural hearing loss).

A 2009 study which followed 189 patients found no excess mortality despite the increased risk of pancreatic cancer.