Type A Niemann–Pick disease (about 85% of cases) has an extremely poor prognosis, with most cases being fatal by the age of 18 months. Type B (adult onset) and type C (mutation affecting a different molecule) Niemann–Pick diseases have a better prognosis.

As of 2010, even with the best care, children with infantile Tay–Sachs disease usually die by the age of 4.

Ashkenazi Jews have a high incidence of Tay–Sachs and other lipid storage diseases. In the United States, about 1 in 27 to 1 in 30 Ashkenazi Jews is a recessive carrier. The disease incidence is about 1 in every 3,500 newborn among Ashkenazi Jews. French Canadians and the Cajun community of Louisiana have an occurrence similar to the Ashkenazi Jews. Irish Americans have a 1 in 50 chance of being a carrier. In the general population, the incidence of carriers as heterozygotes is about 1 in 300. The incidence is approximately 1 in 320,000 newborns in the general population in United States.

Three general classes of theories have been proposed to explain the high frequency of Tay–Sachs carriers in the Ashkenazi Jewish population:

- Heterozygote advantage. When applied to a particular allele, this theory posits that mutation carriers have a selective advantage, perhaps in a particular environment.

- Reproductive compensation. Parents who lose a child because of disease tend to "compensate" by having additional children to replace them. This phenomenon may maintain and possibly even increase the incidence of autosomal recessive disease.

- Founder effect. This hypothesis states that the high incidence of the 1278insTATC chromosomes is the result of an elevated allele frequency that existed by chance in an early founder population.

Tay–Sachs disease was one of the first genetic disorders for which epidemiology was studied using molecular data. Studies of Tay–Sachs mutations using new molecular techniques such as linkage disequilibrium and coalescence analysis have brought an emerging consensus among researchers supporting the founder effect theory.

The incidence among Ashkenazi Jews is estimated to be about one in 40,000 for type A of Niemann–Pick disease. The incidence of both Niemann–Pick disease types A and B in all other populations is estimated to be one in 250,000. The incidence of Niemann–Pick disease type C is estimated to be one in 150,000.

Niemann–Pick Type A, the most common type, occurs in infants and is characterized by jaundice, an enlarged liver, failure to thrive, progressive deterioration of the nervous system and profound brain damage. Children affected by Niemann Pick Type A rarely live beyond 18 months. Niemann–Pick Type A occurs more frequently among individuals of Ashkenazi (eastern and central European) Jewish descent than in other ethnicities. The incidence within the Ashkenazi population is approximately 1 in 40,000 people. The incidence for other populations is 1 in 250,000 people.

Niemann–Pick Type B involves an enlarged liver and spleen hepatosplenomegaly, growth retardation, and problems with lung function including frequent lung infections. Other signs include blood abnormalities such as abnormal cholesterol and lipid levels, and low numbers of blood cells involved in clotting (platelets). The brain is not affected in Type B and the disease often presents in the pre-teen years.

Niemann–Pick type C is a lysosomal storage disease associated with mutations in NPC1 and NPC2 genes. Niemann–Pick type C affects an estimated 1:150,000 people. Approximately 50% of cases present before 10 years of age, but manifestations may first be recognized as late as the sixth decade.

The symptoms of LSD vary, depending on the particular disorder and other variables such as the age of onset, and can be mild to severe. They can include developmental delay, movement disorders, seizures, dementia, deafness, and/or blindness. Some people with LSDhave enlarged livers (hepatomegaly) and enlarged spleens (splenomegaly), pulmonary and cardiac problems, and bones that grow abnormally.

Lysosomal storage diseases (LSDs; ) are a group of about 50 rare inherited metabolic disorders that result from defects in lysosomal function. Lysosomes are sacs of enzymes within cells that digest large molecules and pass the fragments on to other parts of the cell for recycling. This process requires several critical enzymes. If one of these enzymes is defective, because of a mutation, the large molecules accumulate within the cell, eventually killing it.

Lysosomal storage disorders are caused by lysosomal dysfunction usually as a consequence of deficiency of a single enzyme required for the metabolism of lipids, glycoproteins (sugar-containing proteins), or so-called mucopolysaccharides. Individually, LSDs occur with incidences of less than 1:100,000; however, as a group, the incidence is about 1:5,000 - 1:10,000. Most of these disorders are autosomal recessively inherited such as Niemann–Pick disease, type C, but a few are X-linked recessively inherited, such as Fabry disease and Hunter syndrome (MPS II).

The lysosome is commonly referred to as the cell's recycling center because it processes unwanted material into substances that the cell can use. Lysosomes break down this unwanted matter by enzymes, highly specialized proteins essential for survival. Lysosomal disorders are usually triggered when a particular enzyme exists in too small an amount or is missing altogether. When this happens, substances accumulate in the cell. In other words, when the lysosome does not function normally, excess products destined for breakdown and recycling are stored in the cell.

Like other genetic disorders, individuals inherit lysosomal storage diseases from their parents. Although each disorder results from different gene mutations that translate into a deficiency in enzyme activity, they all share a common biochemical characteristic – all lysosomal disorders originate from an abnormal accumulation of substances inside the lysosome.

LSDs affect mostly children and they often die at a young and unpredictable age, many within a few months or years of birth. Many other children die of this disease following years of suffering from various symptoms of their particular disorder.

Sphingolipidoses (singular "sphingolipidosis") are a class of lipid storage disorders relating to sphingolipid metabolism. The main members of this group are Niemann–Pick disease, Fabry disease, Krabbe disease, Gaucher disease, Tay–Sachs disease and metachromatic leukodystrophy. They are generally inherited in an autosomal recessive fashion, but notably Fabry disease is X-linked recessive. Taken together, sphingolipidoses have an incidence of approximately 1 in 10,000, but substantially more in certain populations such as Ashkenazi Jews. Enzyme replacement therapy is available to treat mainly Fabry disease and Gaucher disease, and people with these types of sphingolipidoses may live well into adulthood. The other types are generally fatal by age 1 to 5 years for infantile forms, but progression may be mild for juvenile- or adult-onset forms.

Many lipid storage disorders can be classified into the subgroup of sphingolipidoses, as they relate to sphingolipid metabolism. Members of this group include Niemann-Pick disease, Fabry disease, Krabbe disease, Gaucher disease, Tay-Sachs disease, Metachromatic leukodystrophy, multiple sulfatase deficiency and Farber disease. They are generally inherited in an autosomal recessive fashion, but notably Fabry disease is X-linked. Taken together, sphingolipidoses have an incidence of approximately 1 in 10,000. Enzyme replacement therapy is available to treat mainly Fabry disease and Gaucher disease, and people with these types of sphingolipidoses may live well into adulthood. The other types are generally fatal by age 1 to 5 years for infantile forms, but progression may be mild for juvenile- or adult-onset forms.

Some of the sphingolipidoses may alternatively be classified into either GM1 gangliosidoses or GM2 gangliosidoses. Tay–Sachs disease belongs to the latter.

Other lipid storage disorders that are generally not classified as sphingolipidoses include fucosidosis, Schindler disease and Wolman disease.

The lifespan of patients with NPC is usually related to the age of onset. Children with antenatal or infantile onset usually succumb in the first few months or years of life, whereas adolescent and adult onset forms of Niemann–Pick type C have a more insidious onset and slower progression, and affected individuals may survive to the seventh decade. Adult cases of NPC are being recognized with increasing frequency. It is suspected that many patients affected by NPC are undiagnosed, owing to lack of awareness of the disease and the absence of readily available screening or diagnostic tests. For the same reasons the diagnosis is often delayed by many years.

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

Due to the wide range of genetic disorders that are presently known, diagnosis of a genetic disorder is widely varied and dependent of the disorder. Most genetic disorders are diagnosed at birth or during early childhood, however some, such as Huntington's disease, can escape detection until the patient is well into adulthood.

The basic aspects of a genetic disorder rests on the inheritance of genetic material. With an in depth family history, it is possible to anticipate possible disorders in children which direct medical professionals to specific tests depending on the disorder and allow parents the chance to prepare for potential lifestyle changes, anticipate the possibility of stillbirth, or contemplate termination. Prenatal diagnosis can detect the presence of characteristic abnormalities in fetal development through ultrasound, or detect the presence of characteristic substances via invasive procedures which involve inserting probes or needles into the uterus such as in amniocentesis.

Progressive myoclonus epilepsy (PME) is a rare epilepsy syndrome caused by a variety of genetic disorders. The syndrome includes myoclonic seizures and tonic-clonic seizures together with progressive neurological decline.

PME accounts for less than 1% of epilepsy cases at specialist centres. The incidence and prevalence of PME is unknown, but there are considerable geography and ethnic variations amongst the specific genetic disorders. One cause, Unverricht Lundborg Disease, has an incidence of at least 1:20,000 in Finland.

Pick's disease is a term that can be used in two different ways. It has traditionally been used as a term for a group of neurodegenerative diseases with symptoms attributable to frontal and temporal lobe dysfunction. Common symptoms that are noticed early are personality and emotional changes, as well as deterioration of language. This condition is now more commonly called frontotemporal dementia by professionals, and the use of "Pick's disease" as a clinical diagnosis has fallen out of fashion. The second use of the term (and the one now used among professionals) is to mean a specific pathology that is one of the causes of frontotemporal lobar degeneration. These two uses have previously led to confusion among professionals and patients and so its use should be restricted to the specific pathological subtype described below. It is also known as Pick disease and PiD (not to be confused with pelvic inflammatory disease (PID) or Parkinson's disease (PD)). A defining characteristic of the disease is build-up of tau proteins in neurons, accumulating into silver-staining, spherical aggregations known as "Pick bodies".

Frontotemporal lobar degeneration (FTLD) is a pathological process that occurs in frontotemporal dementia. It is characterized by atrophy in the frontal lobe and temporal lobe of the brain, with sparing of the parietal and occipital lobes.

Common proteinopathies that are found in FTLD include the accumulation of Tau proteins and TARDBPs. Mutations in the C9orf72 gene have been established as a major genetic contribution of FTLD, although defects in the GRN and MAPT genes are also associated with it.

While other pathologies causing frontotemporal lobar degeneration are associated with a genetic cause, evidence is not conclusive in modern research on whether classical Pick's disease pathology has or does not have a direct genetic link, or whether it has been shown to run in families or certain ethnic or gender specific subgroups. However, excess of a protein called β-amyloid (Aβ) in neural cells has been linked to neural degeneration. A buildup of Aβ within cells causes inflammation, leading to cell destruction by the immune system. Proteins associated with Pick's disease are present in all nerve cells, but those with the disease have an abnormal amount.

Mutations in the tau gene (MAPT) have been associated with this disease.

There are 3 main histological subtypes found at post-mortem:

- FTLD-tau is characterised by tau positive inclusions often referred to as Pick-bodies. Examples of FTLD-tau include; Pick's disease, corticobasal degeneration, progressive supranuclear palsy.

- FTLD-TDP (or FTLD-U ) is characterised by ubiquitin and TDP-43 positive, tau negative, FUS negative inclusions. The pathological histology of this subtype is so diverse it is subdivided into four subtypes based on the detailed histological findings:

Two physicians independently categorized the various forms of TDP-43 associated disorders. Both classifications were considered equally valid by the medical community, but the physicians in question have jointly proposed a compromise classification to avoid confusion.

- FTLD-FUS; which is characterised by FUS positive cytoplasmic inclusions, intra nuclear inclusions, and neuritic threads. All of which are present in the cortex, medulla, hippocampus, and motor cells of the spinal cord and XIIth cranial nerve.

Dementia lacking distinctive histology (DLDH) is a rare and controversial entity. New analyses have allowed many cases previously described as DLDH to be reclassified into one of the positively defined subgroups.

There are drugs that can increase serum HDL such as niacin or gemfibrozil. While these drugs are useful for patients with hyperlipidemia, Tangier's disease patients do not benefit from these pharmaceutical interventions.

Therefore, the only current treatment modality for Tangier's disease is diet modification. A low-fat diet can reduce some of the symptoms, especially those involving neuropathies.

Mutations to chromosome 9q31 lead to a defective ABCA1 transporter. These mutations prevent the ABCA1 protein from effectively transporting cholesterol and phospholipids out of cells for pickup by ApoA1 in the bloodstream. This inability to transport cholesterol out of cells leads to a deficiency of high-density lipoproteins in the circulation, which is a risk factor for coronary artery disease. Additionally, the buildup of cholesterol in cells can be toxic, causing cell death or impaired function. These combined factors lead to the signs and symptoms of Tangier disease.

This condition is inherited in an autosomal recessive pattern, meaning that for the phenotype to appear, two copies of the gene must be present in the genotype. Most often, the parents of an individual with an autosomal recessive disorder are carriers of one copy of the altered gene but do not show signs and symptoms of the disorder.

Psycho-organic syndrome is typically diagnosed in individuals following 5–10 years of consistent exposure to chemicals like xylene, toluene, and styrene, which are generally found in paint, plastic and degreasing products.

Patients work and environmental history must be evaluated for exposure to organic chemicals. A traumatic brain injury may also lead to POS.

Consistent intoxication (ie excess use of alcohol and drugs) may also cause a lesion in the brain, eventually leading to POS.

Lyme diseae, the great "new imitator", has been known to mimick depression, scizophreania, depersonazlization disorder, and obsessive-complsive disorder.

Although the cause varies by each individual case, localization of the atrophy in the brain can occur due to aging and without external causes, linked to hereditary.

Prevention includes proper and regular use of Preventive Personal Equipment (PPE) in work environments that involve organic chemicals and limiting alcohol and drug substance intake.

Currently, there is no cure for FTD. Treatments are available to manage the behavioral symptoms. Disinhibition and compulsive behaviors can be controlled by selective serotonin reuptake inhibitors (SSRIs). Although Alzheimer's and FTD share certain symptoms, they cannot be treated with the same pharmacological agents because the cholinergic systems are not affected in FTD.

Because FTD often occurs in younger people (i.e. in their 40's or 50's), it can severely affect families. Patients often still have children living in the home. Financially, it can be devastating as the disease strikes at the time of life that often includes the top wage-earning years.

Personality changes in individuals with FTD are involuntary. Managing the disease is unique to each individual, as different patients with FTD will display different symptoms, sometimes of rebellious nature.