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

Hand-foot-genital syndrome is inherited in an autosomal dominant manner. The proportion of cases caused by de novo mutations is unknown because of the small number of individuals described. If a parent of the proband is affected, the risk to the siblings is 50%. When the parents are clinically unaffected, the risk to the sibs of a proband appears to be low. Each child of an individual with HFGS has a 50% chance of inheriting the mutation. Prenatal testing may be available through laboratories offering custom prenatal testing for families in which the disease-causing mutation has been identified in an affected family member.

Although no cause has been officially confirmed, researchers speculate the disease might result from a genetic mutation that sporadically occurs for unknown reasons.

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

Diagnosis is based on physical examination including radiographs of the hands and feet and imaging studies of the kidneys, bladder, and female reproductive tract. HOXA13 is the only gene known to be associated with HFGS. Approximately 60% of mutations are polyalanine expansions. Molecular genetic testing is clinically available.

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.

Genitopatellar Syndrome is an autosomal dominant inheritance where the mutation in the KAT6B causes the syndrome. The KAT6B gene is responsible for making an enzyme called histone acetyltransferase which functions in regulating and making of histone which are proteins that attach to DNA and give the chromosomes their shape. The function of histone acetyltransferase produced from KAT6B is unknown but it is considered as a regulator of early developments. There is little known about how the mutation in the KAT6B causes the syndrome but researchers suspects that the mutations occur near the end of the KAT6B gene and causes it to produce shortened acetyltransferase enzyme. The shortened enzyme alters the regulation of other genes. On the other hand, the mutation of KAT6B leading to the specific features of genitopatellar syndrome is still not surely proven.

BPES is very rare: only 50-100 cases have been described. It affects slightly more males than females.

Until recently, the medical literature did not indicate a connection among many genetic disorders, both genetic syndromes and genetic diseases, that are now being found to be related. As a result of new genetic research, some of these are, in fact, highly related in their root cause despite the widely varying set of medical symptoms that are clinically visible in the disorders. Ellis–van Creveld syndrome is one such disease, part of an emerging class of diseases called ciliopathies. The underlying cause may be a dysfunctional molecular mechanism in the primary cilia structures of the cell, organelles which are present in many cellular types throughout the human body. The cilia defects adversely affect "numerous critical developmental signaling pathways" essential to cellular development and thus offer a plausible hypothesis for the often multi-symptom nature of a large set of syndromes and diseases. Known ciliopathies include primary ciliary dyskinesia, Bardet–Biedl syndrome, polycystic kidney and liver disease, nephronophthisis, Alstrom syndrome, Meckel–Gruber syndrome and some forms of retinal degeneration.

Weyers acrofacial dysostosis is due to another mutation in the EVC gene and hence is allelic with Ellis–van Creveld syndrome.

Fraser syndrome is a disorder that affects the development of the child prior to birth. Infants born with Fraser syndrome often have eyes that are malformed and completely covered by skin. Also the child is born with fingers and toes that are fused together along with abnormalities within the urine tract. As this disorder relates to vaginal atresia, infants born with Fraser syndrome are also born with malformations in their genitals.

The incidence is estimated to range from 0.1–1.2 per 10,000 live births, though the true incidence is unknown. As of 2005, the highest prevalence was found in Canada and estimated at 1 in 8,500 live births.

In 2008 researchers found autosomal dominant mutations in the RET and GDNF genes to be linked to renal agenesis in unrelated stillborn fetuses through PCR and direct sequence analysis . In the study, DNA from 33 stillborn fetuses were sequenced for mutations in RET, GDNF and GFRA1. Nineteen of the fetuses had BRA, ten had URA and 4 had congenital renal dysplasia. Seven of the 19 BRA fetuses were found to have a mutation in the RET gene (37%), while two of the ten URA fetuses did (20%). One of the URA fetuses had two RET mutations and one GDNF mutation. There were no GFRA1 mutations found.

However, the results of Skinner et al. study were questioned by a more recent study with a larger number of cases . In this study 105 fetuses were analyzed. Sixty-five fetuses had BRA while 24 had URA with an abnormal contralateral kidney. Mutations in the RET gene were only found in seven of the fetuses (6.6%).

In 2014 researchers found autosomal recessive mutations in ITGA8 in three members of two unrelated families utilizing Exome Sequencing . One of the families was consanguineous.

In 2017 researchers identified heritable autosomal dominant mutations in the gene GREB1L in two unrelated families as being the cause of both BRA and URA utilizing Exome Sequencing and direct sequencing analysis . This is the first reported genetic lesion implicated in the activation of Retinoic Acid Receptor (RAR) Targets that has been associated with renal agenesis in humans. The researchers found two different GREB1L mutations, each being unique to their respective pedigrees. In total, there were 23 individuals analyzed between the two families, four of which had BRA and five of which had URA. GREB1L mutations were identified in all of the affected individuals as well as in three unaffected family members, demonstrating incomplete penetrance and variable expressivity.

There are several hundred to perhaps several thousand genes that, if they had the right kind of mutation, could lead to renal agenesis in humans. It is possible that each individual or family experiencing renal agenesis has a unique gene or genetic mutation causing the condition due to the fact that there are so many genes that are critical to proper renal development. See Rosenblum S et al. for an excellent review of Congenital abnormalities of the Kidney and Urinary Tract

Chromosomal anomalies have been associated with BRA in certain cases (chromosomes 1, 2, 5 and 21), but these anomalies were not inherited and have not been observed in subsequent cases. Additionally, neither extreme substance abuse or environmental factors (high power line, mercury, ground water issues, etc.) have been reported to be linked to an increased incidence of BRA or other cause of Potter sequence. However, renal agenesis and other causes of oligohydramnios sequence have been linked to a number of other conditions and syndromes to include Down syndrome, Kallmann syndrome, branchio-oto-renal syndrome and others.

Aphalangy, hemivertebrae and urogenital-intestinal dysgenesis is an extremely rare syndrome, described only in three siblings. It associates hypoplasia or aplasia of phalanges of hands and feet, hemivertebrae and various urogenital and/or intestinal abnormalities. Intrafamilial variability is important as one sister had lethal abnormalities (Potter sequence and pulmonary hypoplasia), while her affected brother was in good health with normal psychomotor development at 6 months of age. Prognosis seems to depend mainly on the severity of visceral malformations. Etiology and inheritance remain unknown.

Barakat syndrome, is a rare disease characterized by hypoparathyroidism, sensorineural deafness and renal disease, and hence also known as HDR syndrome. It was first described by Amin J. Barakat et al. in 1977.

Bardet-Biedl syndrome (BBS) is a cliopathic human genetic disorder that can affect various parts of the body. Parts of the urogenital system where the effects of BBS are seen include: ectopic urethra, renal failure, uterus duplex, hypogonadism, septate vagina, and hypoplasia of the fallopian tubes, uterus, ovaries. Some of the common characteristics associated with this syndrome include intellectual disorders, loss of vision, kidney problems, and obesity.

The mechanism that causes BBS is still remains unclear. Mutations in more than 20 genes can cause BBS and is an inherited recessive condition. Some of the gene mutations that occur in BBS are listed below:

"BBS1, BBS2, ARL6 (BBS3), BBS4, BBS5, MKKS (BBS6), BBS7, TTC8 (BBS8), BBS9, BBS10, TRIM32 (BBS11), BBS12, MKS1 (BBS13), CEP290 (BBS14), WDPCP (BBS15), SDCCAG8 (BBS16), LZTFL1 (BBS17), BBIP1 (BBS18), IFT27 (BBS19), IFT72 (BBS20)", and "C8ORF37(BBS21") The majority of the genes that are related to BBS encode proteins which are called cilia and basal bodies, which are related structures.

Ellis–van Creveld syndrome often is the result of founder effects in isolated human populations, such as the Amish and some small island inhabitants. Although relatively rare, this disorder does occur with higher incidence within founder-effect populations due to lack of genetic variability. Observation of the inheritance pattern has illustrated that the disease is autosomal recessive, meaning that both parents have to carry the gene in order for an individual to be affected by the disorder.

Ellis–van Creveld syndrome is caused by a mutation in the "EVC" gene, as well as by a mutation in a nonhomologous gene, "EVC2", located close to the EVC gene in a head-to-head configuration. The gene was identified by positional cloning. The EVC gene maps to the chromosome 4 short arm (4p16). The function of a healthy EVC gene is not well understood at this time.

Renal agenesis is a medical condition in which one (unilateral) or both (bilateral) fetal kidneys fail to develop.

Unilateral and Bilateral Renal Agenesis in humans, mice and zebra fish has been linked to mutations in the gene GREB1L. It has also been associated with mutations in the genes "RET" or "UPK3A". in humans (see Rosenblum et al 2017 for review) and mice respectively.

Blepharophimosis, ptosis, epicanthus inversus syndrome or BPES is a rare disease characterized by the conditions it is named after: blepharophimosis, ptosis, and epicanthus inversus.

The course of HPS has been mild in rare instances of the disorder, however, the general prognosis is still considered to be poor.

The disease can cause dysfunctions of the lungs, intestine, kidneys, and heart. The major complication of most forms of the disorder is pulmonary fibrosis, which typically exhibits in patients ages 40–50 years. This is a fatal complication seen in many forms of HPS, and is the usual cause of death from the disorder. HPS patients who develop pulmonary fibrosis typically have type 1 or type 4.

Genitopatellar syndrome is a rare disorder with characteristic craniofacial features, congenital flexion contractures of the lower limbs, absent or abnormal patellae, urogenital anomalies, and severe psychomotor retardation.

In 2012, it was shown that mutations in the gene KAT6B cause the syndrome.

Rudiger syndrome is a congenital disorder characterized by the association of severe growth retardation with abnormalities of the extremities, urogenital abnormalities and facial abnormalities. It has been described in a family where an affected brother and sister died as infants. Both autosomal recessive and autosomal dominant inheritance have been suggested with the disorder.

The features ectrodactyly, ectodermal dysplasia and cleft palate have been described with Rudiger syndrome, giving it the rarely used designation "EEC syndrome". However, this is not to be confused with the formal EEC syndrome associated with chromosome 7.

It was characterized in 1971.

HPS is one of the rare lung diseases currently being studied by The Rare Lung Diseases Consortium (RLDC). The RLDC is part of the Rare Diseases Clinical Research Network (RDCRN), an initiative of the Office of Rare Diseases Research (ORDR), of the National Center for Advancing Translational Sciences (NCATS). The RLDC is dedicated to developing new diagnostics and therapeutics for patients with rare lung diseases, through collaboration between the NIH, patient organizations and clinical investigators.

The incidence of Fraser syndrome is 0.043 per 10,000 live born infants and 1.1 in 10,000 stillbirths, making it a rare syndrome.

When an infant is born with PSH, the most difficult management decision has often been the sex assignment, since genitalia with this degree of ambiguity do not resemble either sex very well with respect to looks or function. Many infants with PPHS have been assigned and raised as female despite presence of testes and XY chromosomes.

Nearly all parents of infants with PPSH are offered surgical reconstruction, to either further masculinize or feminize the external genitalia.

Treatment with testosterone postnatally does not close the urethra or change the malformation, but in some cases may enlarge the penis slightly.

Sakati–Nyhan–Tisdale syndrome, also called acrocephalopolysyndactyly type III, is a rare genetic disorder that has been associated with abnormalities in the bones of the legs, congenital heart defects and craniofacial defects. The syndrome belongs to a group of rare genetic disorders known as acrocephalopolysyndactyly or ACPS, for short.