Though the prevalence of Angelman syndrome is not precisely known, there are some estimates. The best data available are from studies of school age children, ages 6–13 years, living in Sweden and from Denmark where the diagnosis of AS children in medical clinics was compared to an 8-year period of about 45,000 births. The Swedish study showed an AS prevalence of about 1/20,000 and the Danish study showed a minimum AS prevalence of about 1/10,000.

Williams syndrome is a microdeletion syndrome caused by the spontaneous deletion of genetic material from the region q11.23 of one member of the pair of chromosome 7, so that the person is hemizygous for those genes. The deleted region includes more than 25 genes, and researchers believe that being hemizygous for these genes probably contributes to the characteristic features of this syndrome. "CLIP2", "ELN", "GTF2I", "GTF2IRD1", and "LIMK1" are among the genes that are typically deleted from one chromosome in people with Williams syndrome. Researchers have found this hemizygosity for the "ELN" gene, which codes for the protein elastin, is associated with the connective-tissue abnormalities and cardiovascular disease (specifically supravalvular aortic stenosis and supravalvular pulmonary stenosis) found in many people with this syndrome. The insufficient supply of elastin may also be the cause of full cheeks, harsh or hoarse voice, hernias and bladder diverticula often found in those with Williams syndrome. Studies suggest that hemizygosity in "LIMK1", "GTF2I", "GTF2IRD1", and perhaps other genes may help explain the characteristic difficulties with visual–spatial tasks. Additionally, there is evidence that the hemizygosity in several of these genes, including "CLIP2", may contribute to the unique behavioral characteristics, learning disabilities, and other cognitive difficulties seen in Williams syndrome.

The severity of the symptoms associated with Angelman syndrome varies significantly across the population of those affected. Some speech and a greater degree of self-care are possible among the least profoundly affected. Walking and the use of simple sign language may be beyond the reach of the more profoundly affected. Early and continued participation in physical, occupational (related to the development of fine-motor control skills), and communication (speech) therapies are believed to significantly improve the prognosis (in the areas of cognition and communication) of individuals affected by AS. Further, the specific genetic mechanism underlying the condition is thought to correlate to the general prognosis of the affected person. On one end of the spectrum, a mutation to the UBE3A gene is thought to correlate to the least affected, whereas larger deletions on chromosome 15 are thought to correspond to the most affected.

The clinical features of Angelman syndrome alter with age. As adulthood approaches, hyperactivity and poor sleep patterns improve. The seizures decrease in frequency and often cease altogether and the EEG abnormalities are less obvious. Medication is typically advisable to those with seizure disorders. Often overlooked is the contribution of the poor sleep patterns to the frequency and/or severity of the seizures. Medication may be worthwhile to help deal with this issue and improve the prognosis with respect to seizures and sleep. Also noteworthy are the reports that the frequency and severity of seizures temporarily escalate in pubescent Angelman syndrome girls, but do not seem to affect long-term health.The facial features remain recognizable with age, but many adults with AS look remarkably youthful for their age.

Puberty and menstruation begin at around the average age. Sexual development is thought to be unaffected, as evidenced by a single reported case of a woman with Angelman syndrome conceiving a female child who also had Angelman syndrome.

The majority of those with AS achieve continence by day and some by night. Angelman syndrome is not a degenerative syndrome, and thus people with AS may improve their living skills with support.

Dressing skills are variable and usually limited to items of clothing without buttons or zippers. Most adults can eat with a knife or spoon and fork, and can learn to perform simple household tasks. General health is fairly good and life-span near average. Particular problems which have arisen in adults are a tendency to obesity (more in females), and worsening of scoliosis if it is present. The affectionate nature which is also a positive aspect in the younger children may also persist into adult life where it can pose a problem socially, but this problem is not insurmountable.

Williams syndrome has historically been estimated to occur in roughly 1 in every 20,000 live births. However, more recent epidemiological studies have placed the occurrence rate at closer to 1 in every 7,500 live births, a significantly larger prevalence. As an increasing body of evidence suggests that Williams syndrome is more common than originally noted (approximately 6% of all genetic cases of developmental disability), researchers have begun to theorize past under-diagnosis of the syndrome. One theorized reason for the increase in epidemiological estimates is that there exists a substantial minority of individuals with the genetic markers of Williams syndrome who lack the characteristic facial features or the diminished IQ considered to be diagnostic of the syndrome, who often are not immediately recognized as people with the syndrome.

22q11.2 deletion syndrome was estimated to affect between one in 2000 and one in 4000 live births. This estimate is based on major birth defects and may be an underestimate, because some individuals with the deletion have few symptoms and may not have been formally diagnosed. It is one of the most common causes of mental retardation due to a genetic deletion syndrome.

The prevalence of 22q11.2DS has been expected to rise because of multiple reasons: (1) Thanks to surgical and medical advances, an increasing number of people are surviving heart defects associated with the syndrome. These individuals are in turn having children. The chances of a 22q11.2DS patient having an affected child is 50% for each pregnancy; (2) Parents who have affected children, but who were unaware of their own genetic conditions, are now being diagnosed as genetic testing become available; (3) Molecular genetics techniques such as FISH (fluorescence in situ hybridization) have limitations and have not been able to detect all 22q11.2 deletions. Newer technologies have been able to detect these atypical deletions.

Recently, the syndrome has been estimated to affect up to one in 2000 live births. Testing for 22q11.2DS in over 9500 pregnancies revealed a prevalence rate of 1/992.

PWS affects approximately 1 in 10,000 to 1 in 25,000 newborns. There are more than 400,000 people who live with PWS around the world.

PWS is commonly associated with development of strabismus. In one study, over 50% of patients had strabismus, mainly esotropia.

The features of this syndrome vary widely, even among members of the same family, and affect many parts of the body. Characteristic signs and symptoms may include birth defects such as congenital heart disease, defects in the palate, most commonly related to neuromuscular problems with closure (velopharyngeal insufficiency, or VPI), learning disabilities, mild differences in facial features, and recurrent infections. Infections are common in children due to problems with the immune system's T-cell-mediated response that in some patients is due to an absent or hypoplastic thymus. 22q11.2 deletion syndrome (22q11.2DS) may be first spotted when an affected newborn has heart defects or convulsions from hypocalcemia due to malfunctioning parathyroid glands and low levels of parathyroid hormone (parathormone).

Affected individuals may also have other kinds of birth defects including kidney abnormalities and significant feeding difficulties as babies. Gastrointestinal issues are also very

common in this patient population. Digestive motility issues may result in constipation. Disorders such as hypothyroidism and hypoparathyroidism or thrombocytopenia (low platelet levels), and psychiatric illnesses are common late-occurring features.

Microdeletions in chromosomal region 22q11.2 are associated with a 20 to 30-fold increased risk of schizophrenia. Studies provide various rates of 22q11.2DS in schizophrenia, ranging from 0.5 to 2.0% and averaging about 1.0%, compared with the overall estimated 0.025% risk of the 22q11.2DS in the general population.

Salient features can be summarized using the mnemonic "CATCH-22" to describe 22q11.2DS, with the 22 signifying the chromosomal abnormality is found on the 22nd chromosome, as below:

- Cardiac abnormality (commonly interrupted aortic arch, truncus arteriosus and tetralogy of Fallot)

- Abnormal facies

- Thymic aplasia

- Cleft palate

- Hypocalcemia/hypoparathyroidism

Some experts support changing the name of both DiGeorge and velocardiofacial syndromes to CATCH-22. The International 22q11.2 Foundation, through its Same Name Campaign, advocates for the consistent use of 22q11.2 deletion syndrome.

Individuals with a 22q11.2 deletion can have many possible features, ranging in number of associated features and from the mild to the very serious. Symptoms shown to be common include:

This syndrome is characterized by incomplete penetrance. Therefore, there is a marked variability in clinical expression between the different patients. This often makes early diagnosis difficult.

The estimated prevalence of Jacobsen syndrome is believed to be approximately 1 out of every 100,000 births. For reasons unknown females are twice as likely to have Jacobsen Syndrome than males. No preference for any race or ethnicity has been reported so far.

The incidence rate of ATR-16 syndrome is not easy to estimate and it is thought to be underdiagnosed. Scientists have described more than 20 cases as of 2013.

Miller-Dieker occurs in less than one in 100000 people and can occur in all races.

Kleefstra syndrome affects males and females equally and approximately, 75% of all documented cases are caused by Eu-HMTase1 disruptions while only 25% are caused by 9q34.3 deletions. There are no statistics on the effect the disease has on life expectancy due to the lack of information available.

Research on the risk for developing schizophrenia in Ashkenazi Jews and other populations showed that 3q29 microdeletion syndrome leads to a significant higher rate of schizophrenia.

Jacobsen Syndrome is caused due to deletion of genetic material from the long arm of chromosome 11. The size of deletion may vary across patients but the deletion always occurs at the end terminal of the q arm of chromosome 11. There are three ways in which the deletion could occur:

de novo deletion- this is a random event that occurred during the formation of the sperm or the egg or during the cell division in the embryonic stage, where genes from chromosome 11 get deleted.

Imbalanced translocation- in this case a parent with balanced translocation or other types of chromosomal rearrangement can pass on these genes to their children which further results in an imbalanced translocation. In this case the affected children have deletions on chromosome 11 as well as some extra genetic material from another chromosome.

Ring chromosome 11- in this case genetic material from both long and short arm of the chromosome get deleted and the remaining part joins together and forms a ring like structure. Here the affected person would have symptoms associated with both 11q and 11p deletion.

While only a few adults have been reported with 2q37 microdeletion syndrome, it is predicted that this number will rise as various research studies continue to demonstrate that most with the disorder do not have a shortened life span.

Affected individuals have a somewhat shortened lifespan. The maximum described lifespan is 67 years. Adults with 13q deletion syndrome often need support services to maintain their activities of daily living, including adult day care services or housing services.

Since the symptoms caused by this disease are present at birth, there is no “cure.” The best cure that scientists are researching is awareness and genetic testing to determine risk factors and increase knowledgeable family planning. Prevention is the only option at this point in time for a cure.

Most individuals with this condition do not survive beyond childhood. Individuals with MDS usually die in infancy and therefore do not live to the age where they can reproduce and transmit MDS to their offspring.

Opitz G/BBB Syndrome is a rare genetic condition caused by one of two major types of mutations: MID1 mutation on the short (p) arm of the X chromosome or a mutation of the 22q11.2 gene on the 22nd chromosome. Since it is a genetic disease, it is an inherited condition. However, there is an extremely wide variability in how the disease presents itself.

In terms of prevention, several researchers strongly suggest prenatal testing for at-risk pregnancies if a MID1 mutation has been identified in a family member. Doctors can perform a fetal sex test through chromosome analysis and then screen the DNA for any mutations causing the disease. Knowing that a child may be born with Opitz G/BBB syndrome could help physicians prepare for the child’s needs and the family prepare emotionally. Furthermore, genetic counseling for young adults that are affected, are carriers or are at risk of carrying is strongly suggested, as well (Meroni, Opitz G/BBB syndrome, 2012). Current research suggests that the cause is genetic and no known environmental risk factors have been documented. The only education for prevention suggested is genetic testing for at-risk young adults when a mutation is found or suspected in a family member.

Smith–Magenis Syndrome (SMS) is a genetic disorder with features including intellectual disability, facial abnormalities, difficulty sleeping, and numerous behavioral problems such as self-harm. Smith–Magenis syndrome affects an estimated between 1 in 15,000 to 1 in 25,000 individuals.

It is a microdeletion syndrome characterized by an abnormality in the short (p) arm of chromosome 17 and is sometimes called the 17p- syndrome.

Due to its recent discovery, there are currently no existing treatments for Kleefstra syndrome.

On several locations in the world people are studying on the subject of 1q21.1 deletion syndrome. The syndrome was identified for the first time with people with heart abnormalities. The syndrome has later been found with patients with autism and schizophrenia. Research is done on patients with a symptom of the syndrome, to find more patients with the syndrome.

There may be a relation between autism and schizophrenia. Literature shows that nine locations have been found on the DNA where the syndromes related to autism or schizophrenia can be found, the so-called "hotspots": 1q21.1, 3q29, 15q13.3, 16p11.2, 16p13.1, 16q21, 17p12, 21q11.2 and 21q13.3. With a number of hotspots both autism and schizophrenia were observed at that location. In other cases, either autism or schizophrenia has been seen.

Statistical research showed that schizophrenia is more common in combination with 1q21.1 deletion syndrome. On the other side, autism is significantly more common with 1q21.1 duplication syndrome. Further research confirmed that the odds on a relation between schizophrenia and deletions at 1q21.1, 3q29, 15q13.3, 22q11.21 en Neurexin 1 (NRXN1) and duplications at 16p11.2 are at 7.5% or higher.

Common variations in the BCL9 gene, which is in the distal area, confer risk of schizophrenia and may also be associated with bipolar disorder and major depressive disorder.

Research is done on 10–12 genes on 1q21.1 that produce DUF1220-locations. DUF1220 is an unknown protein, which is active in the neurons of the brain near the neocortex. Based on research on apes and other mammals, it is assumed that DUF1220 is related to cognitive development (man: 212 locations; chimpanzee: 37 locations; monkey: 30 locations; mouse: 1 location). It appears that the DUF1220-locations on 1q21.1 are in areas that are related to the size and the development of the brain. The aspect of the size and development of the brain is related to autism (macrocephaly) and schizophrenia (microcephaly). It has been proposed that a deletion or duplication of a gene that produces DUF1220-areas might cause growth and development disorders in the brain

Another relation between macrocephaly with duplications and microcephaly with deletions has been seen in research on the HYDIN Paralog or HYDIN2. This part of 1q21.1 is involved in the development of the brain. It is assumed to be a dosage-sensitive gene. When this gene is not available in the 1q21.1 area, it leads to microcephaly. HYDIN2 is a recent duplication (found only in humans) of the HYDIN gene found on 16q22.2.

Research on the genes CHD1L and PRKAB2 within lymphoblast cells lead to the conclusion that anomalies appear with the 1q21.1-deletionsyndrome:

- CHD1L is an enzyme which is involved in untangling the chromatides and the DNA repair system. With 1q21.1 deletion syndrome a disturbance occurs, which leads to increased DNA breaks. The role of CHD1L is similar to that of helicase with the Werner syndrome

- PRKAB2 is involved in maintaining the energy level of cells. With 1q21.1-deletion syndrome this function was attenuated.

GJA5 has been identified as the gene that is responsible for the phenotypes observed with congenital heart diseases on the 1q21.1 location. In case of a duplication of GJA5 tetralogy of Fallot is more common. In case of a deletion other congenital heart diseases than tetralogy of Fallot are more common.

Several researchers around the world are studying on the subject of 1q21.1 duplication syndrome. The syndrome was identified for the first time in people with heart abnormalities. The syndrome was later observed in patients who had autism or schizophrenia.

It appears that there is a relation between autism and schizophrenia. Literature shows that nine locations have been found on the DNA where the syndromes related to autism or schizophrenia can be found, the so-called "hotspots": 1q21.1, 3q29, 15q13.3, 16p11.2, 16p13.1, 16q21, 17p12, 21q11.2 and 21q13.3. With a number of hotspots both autism and schizophrenia were observed at that location. In other cases, either autism or schizophrenia has been seen, while they are searching for the opposite.

Statistical research showed that schizophrenia is significantly more common in combination with 1q21.1 deletion syndrome. On the other side, autism is significantly more common with 1q21.1 duplication syndrome. Similar observations were done for chromosome 16 on 16p11.2 (deletion: autism/duplication: schizophrenia), chromosome 22 on 22q11.21 (deletion (Velo-cardio-facial syndrome): schizophrenia/duplication: autism) and 22q13.3 (deletion (Phelan-McDermid syndrome): schizophrenia/duplication: autism). Further research confirmed that the odds on a relation between schizophrenia and deletions at 1q21.1, 3q29, 15q13.3, 22q11.21 en Neurexin 1 (NRXN1) and duplications at 16p11.2 are at 7.5% or higher.

Common variations in the BCL9 gene, which is in the distal area, confer risk of schizophrenia and may also be associated with bipolar disorder and major depressive disorder.

Research is done on 10-12 genes on 1q21.1 that produce DUF1220-locations. DUF1220 is an unknown protein, which is active in the neurons of the brain near the neocortex. Based on research on apes and other mammals, it is assumed that DUF1220 is related to cognitive development (man: 212 locations; chimpanzee: 37 locations; monkey: 30 locations; mouse: 1 location). It appears that the DUF1220-locations on 1q21.1 are in areas that are related to the size and the development of the brain. The aspect of the size and development of the brain is related to autism (macrocephaly) and schizophrenia (microcephaly). It is assumed that a deletion or a duplication of a gene that produces DUF1220-areas might cause growth and development disorders in the brain

Another relation between macrocephaly with duplications and microcephaly with deletions has been seen in research on the HYDIN Paralog or HYDIN2. This part of 1q21.1 is involved in the development of the brain. It is assumed to be a dosage-sensitive gene. When this gene is not available in the 1q21.1 area it leads to microcephaly. HYDIN2 is a recent duplication (found only in humans) of the HYDIN gene found on 16q22.2.

GJA5 has been identified as the gene that is responsible for the phenotypes observed with congenital heart diseases on the 1q21.1 location. In case of a duplication of GJA5 tetralogy of Fallot is more common. In case of a deletion other congenital heart diseases than tetralogy of Fallot are more common.

Smith–Magenis syndrome is a chromosomal condition related to low copy repeats of specific segments of chromosome 17. Most people with SMS have a deletion of genetic material from a specific region of chromosome 17 (17p11.2). Although this region contains multiple genes, recently researchers discovered that the loss of one particular gene the retinoic acid induced 1 or "RAI1" is responsible for most of the characteristic features of this condition. Also, other genes within the chromosome 17 contribute to the variability and severity of the clinical features. The loss of other genes in the deleted region may help explain why the features of Smith–Magenis syndrome vary among affected individuals. A small percentage of people with Smith–Magenis syndrome have a mutation in the "RAI1" gene instead of a chromosomal deletion.

These deletions and mutations lead to the production of an abnormal or nonfunctional version of the "RAI1" protein. "RAI1" is a transcription factor that regulates the expression of multiple genes, including several that are involved in controlling circadian rhythm, such as "CLOCK". The groups led by James Lupski (Baylor College of Medicine) and Sarah Elsea (Virginia Commonwealth University) are in the process of studying the exact function of this gene in relation to Smith Magenis Syndrome.

SMS is typically not inherited. This condition usually results from a genetic change that occurs during the formation of reproductive cells (eggs or sperm) or in early fetal development. People with Smith–Magenis syndrome most often have no history of the condition in their family.

Currently, research is focusing on identifying the role of the genes on 18p in causing the signs and symptoms associated with deletions of 18p. This will ultimately enable predictive genotyping.

TGIF-Mutations and deletions of this gene have been associated with holoprosencephaly. Penetrance is incomplete, meaning that a deletion of one copy of this gene is not in and of itself sufficient to cause holoprosencephaly. Ten to fifteen percent of people with 18p- have holoprosencephaly, suggesting that other genetic and environmental facts play a role in the etiology of holoprosencephaly in these individuals.