Fucosidosis is an extremely rare disorder first described in 1962 in two Italian siblings who showed progressive intellectual disability and neurological deterioration. The disease itself is extremely rare (less than 100 documented cases) only affecting 1:2,000,000, with most cases being occurring in Italy, Cuba, and the southwest U.S. The disease has three different types. Type 1 and 2 are considered severe, and Type 3 being a mild disease. Symptoms are highly variable with mild cases being able to live to within the third or fourth decade. Type 1 and 2 are both linked with mental retardation. Severe cases can develop life-threatening complications early in childhood.

Because the major accumulating glycoconjugate in fucosidosis patients is the blood group H-antigen, it is intriguing to speculate, but the evidence is not clear at this time, that blood type may affect the course of the disease.

The clinical course of BVVL can vary from one patient to another. There have been cases with progressive deterioration, deterioration followed by periods of stabilization, and deterioration with abrupt periods of increasing severity.

The syndrome has previously been considered to have a high mortality rate but the initial response of most patients to the Riboflavin protocol are very encouraging and seem to indicate a significantly improved life expectancy could be achievable. There are three documented cases of BVVL where the patient died within the first five years of the disease. On the contrary, most patients have survived more than 10 years after the onset of their first symptom, and several cases have survived 20–30 years after the onset of their first symptom.

Families with multiple cases of BVVL and, more generally, multiple cases of infantile progressive bulbar palsy can show variability in age of disease onset and survival. Dipti and Childs described such a situation in which a family had five children that had Infantile PBP. In this family, three siblings showed sensorineural deafness and other symptoms of BVVL at an older age. The other two siblings showed symptoms of Fazio-Londe disease and died before the age of two.

RHBDF2 may also play a role in ovarian epithelial cancer.

Possible associations with gastric cancer and lung cancer have been suggested. Other possible associations include corneal defects, congenital pulmonary stenosis, total anomalous pulmonary venous connection deafness and optic atrophy.

This condition is inherited as an autosomal dominant syndrome and characterized by palmoplantar keratoderma, oral precursor lesions particularly on the gums (leukoplakia) and a high lifetime risk of esophageal cancer (95% develop esophageal cancer by the age of 65). Relapsing cutaneous horns of the lips has been reported in this condition.

There are several types of this condition have been described – epidermolytic (Vörner type) and non-epidermolytic. Another classification divides these into an early onset type (type B) which occurs in the first year of life and is usually benign and a type A tylosis which occurs between the ages of 5 and 15 years and is strongly associated with esophageal cancer.

Cytoglobin gene expression in oesphageal biopsies is significantly reduced (70% reduction) in this condition. The mechanism of this change is not known.

In itself, NSML is not a life-threatening diagnosis, most people diagnosed with the condition live normal lives. Obstructive cardiomyopathy and other pathologic findings involving the cardiovascular system may be a cause of death in those whose cardiac deformities are profound.

The syndrome primarily affects young males. Preliminary studies suggest that prevalence may be 1.8 per 10,000 live male births. 50% of those affected do not live beyond 25 years of age, with deaths attributed to the impaired immune function.

Canine fucosidosis is found in the English Springer Spaniel.

Typically affecting dogs between 18 months and four years, symptoms include:

- Loss of learned behavior

- Change in temperament

- Blindness

- Loss of balance

- Deafness

- Weight loss

- From the onset, disease progress is quick and fatal.

Just like the human version, canine fucosidosis is a recessive disorder and two copies of the gene must be present, one from each parent, in order to show symptoms of the disease.

As of 2015, there are approximately 70 known cases of Brown-Vialetto-Van-Laere syndrome worldwide. BVVL was first described in a Portuguese family, and has since been described in a number of ethnic groups. Reports have shown that BVVL infects females more than males at a rate of 5:1 respectively. However, males usually exhibit more severe symptoms, an earlier onset of deafness and a tendency to die earlier in life.

Keratitis–ichthyosis–deafness syndrome (also known as "Erythrokeratodermia progressiva Burns," "Ichthyosiform erythroderma, corneal involvement, and deafness," and "KID syndrome,") presents at birth/infancy and is characterized by pregressive corneal opacification, either mild generalized hyperkeratosis or discrete erythematous plaques, and neurosensory deafness.

It is caused by a mutation in connexin 26.

Campomelic dysplasia has a reported incidence of 0.05-0.09 per 10000 live births.

In nearly 95% of the cases, death occurs in the neonatal period due to respiratory distress, generally related to small chest size or insufficient development of the trachea and other upper airway structures.

Among survivors of CMD, the skeletal malformations change over time to include worsening scoliosis or kyphosis resulting in decreased trunk size relative to the limb length. Neurological damage is also often seen including mental retardation and deafness. Even among survivors of the prenatal period, CMD patients have shortened life spans due to lifelong respiratory issues. Those patients with ambiguous genitalia or sex reversal at birth, of course, maintain that state, and are either sterile or have reduced fertility.

As its name indicates, a person with the syndrome has one Y chromosome and four X chromosomes on the 23rd pair, thus having 49 chromosomes rather than the normal 46. As with most categories of aneuploidy disorders, 49,XXXXY syndrome is often accompanied by intellectual disability. It can be considered a form of 47, XXY Klinefelter syndrome, or a variant of it.

It is genetic but not hereditary. This means that while the genes of the parents cause the syndrome, there is a small chance of more than one child having the syndrome. The probability of inheriting the disease is about 1%.

The individuals with this syndrome are males, but 49, XXXXX also exists with similar characteristics.

The frequency is unknown, but the disease is considered to be very rare.

There is no known cure for this syndrome. Patients usually need ophthalmic surgery and may also need dental surgery

Genetic counseling and screening of the mother's relatives is recommended.

Langer mesomelic dysplasia (LMD) is a rare congenital disorder characterized by an altered bone formation that causes a severe short and disproportionate stature.

The incidence is less than 1/1.000.000. Fewer than 50 cases have been reported so far.

X-linked recessive chondrodysplasia punctata is a type of chondrodysplasia punctata that can involve the skin, hair, and cause short stature with skeletal abnormalities, cataracts, and deafness.

This condition is also known as arylsulfatase E deficiency, CDPX1, and X-linked recessive chondrodysplasia punctata 1. The syndrome rarely affects females, but they can be carriers of the recessive allele. Although the exact number of people diagnosed with CDPX1 is unknown, it was estimated that 1 in 500,000 have CDPX1 in varying severity. This condition is not linked to a specific ethnicity. The mutation that leads to a deficiency in arylsulfatase E. (ARSE) occurs in the coding region of the gene.Absence of stippling, deposits of calcium, of bones and cartilage, shown on x-ray, does not rule out chondrodysplasia punctata or a normal chondrodysplasia punctata 1 (CDPX1) gene without mutation. Stippling of the bones and cartilage is rarely seen after childhood. Phalangeal abnormalities are important clinical features to look for once the stippling is no longer visible. Other, more severe, clinical features include respiratory abnormalities, hearing loss, cervical spine abnormalities, delayed cognitive development, ophthalmologic abnormalities, cardiac abnormalities, gastroesophageal reflux, and feeding difficulties. CDPX1 actually has a spectrum of severity; different mutations within the CDPX1 gene have different effects on the catalytic activity of the ARSE protein. The mutations vary between missense, nonsense, insertions, and deletions.

In the two predominant mutations of NSML (Y279C and T468M) the mutations cause a loss of catalytic activity of the SHP2 protein (the gene product of the "PTPN11" gene), which is a previously unrecognized behavior for this class of mutations. This interferes with growth factor and related signalling. While further research confirms this mechanism, additional research is needed to determine how this relates to all of the observed effects of NSML.

Campomelic dysplasia (CMD) is a rare genetic disorder characterized by bowing of the long bones and many other skeletal and extraskeletal features.

It is frequently lethal in the neonatal period due to respiratory insufficiency, but the severity of the disease is variable, and some patients survive into adulthood.

The name is derived from the Greek roots "campo" (or "campto"), meaning bent, and "melia", meaning limb.

An unusual aspect of the disease is that up to two-thirds of affected 46,XY genotypic males display a range of Disorders of Sexual Development (DSD) and genital ambiguities or may even develop as normal phenotypic females as in complete 46 XY sex reversal.

An atypical form of the disease with absence of bowed limbs is called, prosaically, acampomelic campomelic dysplasia (ACD) and is found in about 10% of patients, particularly those surviving the neonatal period.

This syndrome is due to mutations in the Nance Horan gene (NHS) which is located on the short arm of the X chromosome (Xp22.13).

One family of 68 individuals over 5 generations was studied and the prevalence of disease among the family members suggests that it is indicative of dominant inheritance that is not sexually linked. This is supported by the fact that the disease failed to skip generations even in the absence of intermarriages and that disease incidence was independent of sex. The current findings suggest that the cause of the disease could be narrowed down to one enzymatic defect that is involved in the development of neuroectodermal tissue, however the exact molecular mechanisms are currently unknown. The other symptoms that arise such as bone defects and diabetes may be secondary to this enzymatic defect.

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

MECP2 Duplication Syndrome (M2DS) is a rare disease that is characterized by severe intellectual disability and impaired motor function. It is an X-linked genetic disorder caused by the overexpression of MeCP2 protein.

Bazex–Dupré–Christol syndrome (also known as "Bazex syndrome", and "follicular atrophoderma and basal cell carcinomas") is a very rare condition inherited in an X-linked dominant fashion. Physical findings typically include follicular atrophoderma, multiple basal cell carcinomas, hypotrichosis, and hypohidrosis.

This condition should not be confused with the unrelated condition acrokeratosis paraneoplastica of Bazex, which may also be referred to Bazex syndrome.

X-linked myotubular myopathy (MTM) is a form of centronuclear myopathy (CNM) associated with myotubularin 1.

Genetically inherited traits and conditions are often referred to based upon whether they are located on the "sex chromosomes" (the X or Y chromosomes) versus whether they are located on "autosomal" chromosomes (chromosomes other than the X or Y). Thus, genetically inherited conditions are categorized as being sex-linked (e.g., X-linked) or autosomal. Females have two X-chromosomes, while males only have a single X chromosome, and a genetic abnormality located on the X chromosome is much more likely to cause clinical disease in a male (who lacks the possibility of having the normal gene present on any other chromosome) than in a female (who is able to compensate for the one abnormal X chromosome).

The X-linked form of MTM is the most commonly diagnosed type. Almost all cases of X-linked MTM occurs in males. Females can be "carriers" for an X-linked genetic abnormality, but usually they will not be clinically affected themselves. Two exceptions for a female with a X-linked recessive abnormality to have clinical symptoms: one is a manifesting carrier and the other is X-inactivation. A manifesting carrier usually has no noticeable problems at birth; symptoms show up later in life. In X-inactivation, the female (who would otherwise be a carrier, without any symptoms), actually presents with full-blown X-linked MTM. Thus, she congenitally presents (is born with) MTM.

Thus, although" MTM1" mutations most commonly cause problems in boys, these mutations can also cause clinical myopathy in girls, for the reasons noted above. Girls with myopathy and a muscle biopsy showing a centronuclear pattern should be tested for "MTM1" mutations.

Many clinicians and researchers use the abbreviations XL-MTM, XLMTM or X-MTM to emphasize that the genetic abnormality for myotubular myopathy (MTM) is X-linked (XL), having been identified as occurring on the X chromosome. The specific gene on the X chromosome is referred to as MTM-1. In theory, some cases of CNM may be caused by an abnormality on the X chromosome, but located at a different site from the gene "MTM1", but currently "MTM1" is the only X-linked genetic mutation site identified for myotubular or centronuclear myopathy. Clinical suspicion for X-linked inheritance would be a disease affecting multiple boys (but no girls) and a pedigree chart showing inheritance only through the maternal (mother’s) side of each generation.

The chronic inflammation present in MWS over time can lead to deafness. In addition, the prolonged inflammation can lead to deposition of proteins in the kidney, a condition known as amyloidosis.