The severity of symptoms of idic(15) vary greatly between individuals. Individuals with idic(15) usually have delays in language development and motor skills such as walking or sitting up. Other traits may include low muscle tone (hypotonia), seizures (>50%), short stature, and intellectual disability. Distinctive facial features associated with idic(15), where present, are usually very subtle but may include epicanthal folds (skin folds at the inner corners of one or both eyes), broad forehead, a flattened nasal bridge, button nose, and a high arched palate (roof of the mouth). Many individuals with idic(15) display features of autism, such as problems with communication and social interactions, obsessional interests (often with interactive mechanisms like wheels, doors or switches), unpredictable sleep cycles (and a reduced need for sleep), and repetitive and stereotyped behaviors (e.g., lining up toys, playing with a toy in the same manner over and over again, hand flapping, rocking back and forth). A high pain threshold is often observed. If speech develops, it is often echolalic but some individuals do grasp some language. With a severely affected person there may be an inability to walk or talk.

The syndrome gets its name from the characteristic cry of affected infants, which is similar to that of a meowing kitten, due to problems with the larynx and nervous system. About 1/3 of children lose the cry by age of 2 years. Other symptoms of cri du chat syndrome may include:

- feeding problems because of difficulty in swallowing and sucking;

- low birth weight and poor growth;

- severe cognitive, speech, and motor delays;

- behavioral problems such as hyperactivity, aggression, outbursts, and repetitive movements;

- unusual facial features which may change over time;

- excessive drooling;

- small head and jaw;

- wide eyes;

- skin tags in front of eyes.

Other common findings include hypotonia, microcephaly, growth retardation, a round face with full cheeks, hypertelorism, epicanthal folds, down-slanting palpebral fissures, strabismus, flat nasal bridge, down-turned mouth, micrognathia, low-set ears, short fingers, single palmar creases, and cardiac defects (e.g., ventricular septal defect [VSD], atrial septal defect [ASD], patent ductus arteriosus [PDA], tetralogy of Fallot). Infertility is not associated with Cri du chat.

It has also been observed that people with the condition have difficulties communicating. While levels of proficiency can range from a few words to short sentences, it is often recommended by medical professionals for the child to undergo some sort of speech therapy/aid with the help of a professional.

Less frequently encountered findings include cleft lip and palate, preauricular tags and fistulas, thymic dysplasia, intestinal malrotation, megacolon, inguinal hernia, dislocated hips, cryptorchidism, hypospadias, rare renal malformations (e.g., horseshoe kidneys, renal ectopia or agenesis, hydronephrosis), clinodactyly of the fifth fingers, talipes equinovarus, pes planus, syndactyly of the second and third fingers and toes, oligosyndactyly, and hyperextensible joints. The syndrome may also include various dermatoglyphics, including transverse flexion creases, distal axial triradius, increased whorls and arches on digits, and a single palmar crease.

Late childhood and adolescence findings include significant intellectual disability, microcephaly, coarsening of facial features, prominent supraorbital ridges, deep-set eyes, hypoplastic nasal bridge, severe malocclusion, and scoliosis.

Affected females reach puberty, develop secondary sex characteristics, and menstruate at the usual time. The genital tract is usually normal in females except for a report of a bicornuate uterus. In males, testes are often small, but spermatogenesis is thought to be normal.

ATR-16 syndrome affects the blood, development, and brain; symptoms vary based on the specific genes deleted on chromosome 16. Because it is so rare, it is difficult to determine the "core" symptoms of the disease. People with ATR-16 have alpha-thalassemia, a blood disorder where there is less normal hemoglobin in the blood than there should be, and the red blood cells are smaller than they should be (microcytic anemia). Affected children have various characteristic physical features, including clubfoot, "locked" little fingers, microcephaly (small head), hypertelorism (widely spaced eyes), broad, prominent nose bridge, downward-slanted palpebral fissures, small ears, retrognathia, and short neck. Children with ATR-16 syndrome also have mild to moderate intellectual disabilities, developmental delays/growth delays, and speech delays. Some children with ATR-16 have seizures, cryptorchidism (undescended testes), or hypospadias.

Coffin–Lowry syndrome is a severe mental retardation associated with abnormalities of:

- Growth

- "In utero" growth is normal but post natal growth is retarded. Patients are sometimes microcephalic.

- Cardio-vascular

- Cardiac abnormalities affect 15% of the patients.

- Skeleton

- Progressive kyphoscoliosis affects 1 in 2 patients. Micrognathia is also associated with this syndrome.

- Patients may also have an underdeveloped upper jaw bone, abnormally prominent brows, or widely spaced eyes.

- Vision and audition

- Auditory abnormalities are frequent and often present. Vision abnormalities are not often present.

Affected individuals present with a broad array of medical and behavioral manifestations (tables 1 and 2). Patients are consistently characterized by global developmental delay, intellectual disability, speech abnormalities, ASD-like behaviors, hypotonia and mild dysmorphic features. Table 1 summarizes the dysmorphic and medical conditions that have been reported in individuals with PMS. Table 2 summarize the psychiatric and neurological associated with PMS. Most of the studies include small samples or relied on parental report or medical record review to collect information, which can account in part for the variability in the presentation of some of the presenting features. Larger prospective studies are needed to further characterize the phenotype.

Table 1: Dysmorphic features and medical comorbid conditions that have been reported in individuals with Phelan McDermid Syndrome.

Table 2: Psychiatric and Neurologic Manifestations associated with Phelan McDermid Syndrome

Following are the features and characteristics that help in spotting this disorder:

- Low birth weight (usually under 5 pounds/2.5 kilograms)

- Delayed growth and small stature

- Developmental delay

- Limb differences (missing limbs or portions of limbs)

- Small head size (microcephaly)

- Thick eyebrows, which typically meet at midline (synophrys)

- Long eyelashes

- Short upturned nose and thin downturned lips

- Long philtrum

- Excessive body hair

- Small hands and feet

- Small widely spaced teeth

- Low-set ears

- Hearing impairments

- Vision abnormalities (e.g., ptosis, nystagmus, high myopia, hypertropia)

- Partial joining of the second and third toes

- Incurved 5th fingers (clinodactyly)

- Gastroesophageal reflux

- Seizures

- Heart defects (e.g., pulmonary stenosis, VSD, ASD, coarctation of the aorta)

- Cleft palate

- Feeding problems

- Hypoplastic genitalia

Children with this syndrome are often found to have long eyelashes, bushy eyebrows and synophrys (joined eyebrows). Body hair can be excessive and affected individuals are often shorter than their immediate family members. They present a characteristic facial phenotype and is recognizable with the Facial Dysmorphology Novel Analysis (FDNA) technology

CdLS can give rise to its own array of complexities. Children with CdLS often suffer from gastrointestinal tract difficulties, particularly gastroesophageal reflux. Vomiting, intermittent poor appetite, constipation, diarrhea or gaseous distention are known to be a regularity in cases where the GI tract problems are acute. Symptoms may range from mild to severe.

CdLS may include behavior problems, including self-stimulation, aggression, self-injury or strong preference to a structured routine. Many children with CdLS exhibit autistic-like behaviors.

Behavior problems in CdLS are not inevitable. Many behavior issues associated with CdLS are reactive (i.e., something happens within the person's body or environment to bring on the behavior) and cyclical (comes and goes). Often, an underlying medical issue causes a change in behavior. Once the medical issue is treated, the behavior diminishes.

The most common characteristics include a distinct craniofacial phenotype (microcephaly, micrognathia, short philtrum, prominent glabella, ocular hypertelorism, dysplastic ears and periauricular tags), growth restriction, intellectual disability, muscle hypotonia, seizures, and congenital heart defects. Less common characteristics include hypospadias, colobomata of the iris, renal anomalies, and deafness. Antibody deficiencies are also common, including common variable immunodeficiency and IgA deficiency. T-cell immunity is normal.

Cri du chat syndrome, also known as chromosome 5p deletion syndrome, 5p− syndrome (pronounced "Five P Minus") or Lejeune’s syndrome, is a rare genetic disorder due to chromosome deletion on chromosome 5. Its name is a French term ("cat-cry" or "call of the cat") referring to the characteristic cat-like cry of affected children. It was first described by Jérôme Lejeune in 1963. The condition affects an estimated 1 in 50,000 live births across all ethnicities and is more common in females by a 4:3 ratio.

Coffin–Lowry syndrome is a genetic disorder that is X-linked dominant and which causes severe mental problems sometimes associated with abnormalities of growth, cardiac abnormalities, kyphoscoliosis, as well as auditory and visual abnormalities.

Though only definitively diagnosable by genetic sequence testing, including a G band analysis, ATR-16 syndrome may be diagnosed from its constellation of symptoms. It must be distinguished from ATR-X syndrome, a very similar disease caused by a mutation on the X chromosome, and cases of alpha-thalassemia that co-occur with intellectual disabilities with no underlying genetic relationship.

22q13 deletion syndrome (spoken as "twenty-two q one three", see Locus (genetics)) is a genetic disorder caused by deletions or rearrangements on the q terminal end (long arm) of chromosome 22. Any abnormal genetic variation in the q13 region that presents with significant manifestations (phenotype) typical of a terminal deletion may be diagnosed as 22q13 deletion syndrome. 22q13 deletion syndrome is often called Phelan-McDermid syndrome (abbreviated PMS). There is disagreement among researchers as to the exact definition of 22q13 deletion syndrome. The Developmental Synaptopathies Consortium defines PMS as being caused by "SHANK3" mutations, a definition that appears to exclude terminal deletions. The requirement to include "SHANK3" in the definition is supported by many, but not by those who first described 22q13 deletion syndrome.

A prototypical terminal deletion of 22q13 can be uncovered by karyotype analysis, but many terminal and interstitial deletions are too small. The availability of DNA microarray technology for revealing multiple genetic problems simultaneously has been the diagnostic tool of choice. The falling cost for whole exome sequencing and, eventually, whole genome sequencing, may replace DNA microarray technology for candidate evaluation. However, fluorescence in situ hybridization (FISH) tests remain valuable for diagnosing cases of mosaicism (mosaic genetics) and chromosomal rearrangements (e.g., ring chromosome, unbalanced chromosomal translocation). Although early researchers sought a monogenic (single gene genetic disorder) explanation, recent studies have not supported that hypothesis (see Etiology, below).

Isodicentric 15, also called idic(15), partial tetrasomy 15q, or inverted duplication 15 (inv dup 15), is a chromosome abnormality in which a child is born with extra genetic material from chromosome 15. People with idic(15) are typically born with 47 chromosomes in their body cells, instead of the normal 46. The extra chromosome is made up of a piece of chromosome 15 that has been duplicated end-to-end like a mirror image. It is the presence of this extra genetic material that is thought to account for the symptoms seen in some people with idic(15). Individuals with idic(15) have a total of four copies of this chromosome 15 region instead of the usual two copies (1 copy each on the maternal and paternal chromosomes).

The syndrome is also often referred to by the broader term "Chromosome 15q11.2-q13.1 Duplication Syndrome", shortened to Dup15q syndrome, a name that is supported and actively promoted by the US-based support organization Dup15q Alliance. Dup15q syndrome is a broader disease term, as it includes both idic(15) and interstitial 15q11.2-q13.1, another type of duplication that causes similar clinical traits.

The extra chromosome is occasionally found in the mosaic state, i.e. some of the cells carry the marker chromosome. However, mostly because of the marker's instability and tendency to be lost during cell division (mitosis), some cells are completely normal with 46 chromosomes. Occasionally, cells may have more than one idic(15), resulting in 48 or 49 chromosomes in all or some of their cells. A similar clinical picture albeit to a milder degree could be expected in individuals that have the extra chromosome 15 material as an interstitial duplication (when the extra piece of chromosome 15 is included "within" the long arm of one of the two copies of chromosome 15, rather than as a small extra 'marker' chromosome) - often abbreviated to int dup(15); the individual thus having 46 chromosomes.

Cornelia de Lange Syndrome (CdLS) is a very rare genetic disorder present from birth, but not always diagnosed at birth. It causes a range of physical, cognitive, and medical challenges and affects both sexes equally. The syndrome is named after Dutch pediatrician Cornelia Catharina de Lange, who described it.

It is often termed Bushy Syndrome and is also known as Amsterdam dwarfism. It is a genetic disorder that can lead to severe developmental anomalies. It affects the physical and intellectual development of a child. Exact incidence is unknown, but it is estimated at 1 in 10,000 to 30,000.

Wolf–Hirschhorn syndrome (WHS), also known as chromosome deletion Dillan 4p syndrome, Pitt–Rogers–Danks syndrome (PRDS) or Pitt syndrome, was first described in 1961 by Americans Herbert L. Cooper and Kurt Hirschhorn and, thereafter, gained worldwide attention by publications by the German Ulrich Wolf, and Hirschhorn and their co-workers, specifically their articles in the German scientific magazine "Humangenetik". It is a characteristic phenotype resulting from a partial deletion of chromosomal material of the short arm of chromosome 4 (del(4p16.3)).

The phenotypic data on 11 patients indicated that cases are not always ascertained for CHD but that CHD was the most common single feature found in 6 out of 11 individuals. Developmental delay and/or learning difficulties were found in 5 out of 11 cases, but one prenatal case was developing normally at 15 months of age (Case 1,). Three other prenatal cases could not yet be reliably assessed. A variable degree of facial dysmorphism was present in 5 out of 11 individuals. Partial toe syndactyly has been found in one mother and son diad and adrenal anomalies in two probands but not in the duplicated mother of one of them. The phenotype is compatible with independent adult life with varying degrees of support.

Duplication of the GATA4 transcription factor () is believed to underlie the congenital heart disease and other genes, common to the duplication and deletion syndromes, can be regarded as candidates for the 8p23.1 duplication syndrome. These include the SOX7 transcription factor () for both CHD and developmental delay and the TNKS gene () for behavioural difficulties. The diaphragmatic hernia found in the 8p23.1 deletion syndrome has not been found in the 8p23.1 duplication syndrome to date.

The duplication may be associated with copy number changes of the adjacent olfactory receptor/defensin repeats (ORDRs) that predispose to the 8p23.1 deletion and duplication syndromes. High total copy numbers of these repeats have been associated with predisposition to psoriasis and low copy number with predisposition to Crohn's disease.

Little is known about the natural history of Roberts syndrome due to its wide clinical variability. The prognosis of the disease depends on the malformations, as the severity of the malformations correlates with survival. The cause of death for most fatalities of Roberts syndrome have not been reported; however, five deaths were reportedly due to infection.

The following are observations that have been made in individuals with cytogenetic findings of PCS/HR or ESCO2 mutations:

- The symptom of prenatal growth retardation is the most common finding and can be moderate to severe. Postnatal growth retardation can also be moderate to severe and correlates with the degree of severity of limb and craniofacial malformations.

- In limb malformations, the upper limbs are typically more severely affected than the lower limbs. There have been many cases of only upper limb malformation.

- In hand malformations, the thumb is most often affected, followed by the fifth finger (the little finger). In severe cases, the patient may only have three fingers and in rare cases only one.

- In craniofacial malformations, mildly affected individuals will have no abnormalities of the palate. The most severely affected will have a fronto-ethmoid-nasal-maxillary encephalocele.

- The severity of limb malformations and craniofacial malformations is correlated.

- Other abnormalities can occur in different parts of the body, including:

- Heart- atrial septal defects, ventricular septal defects, patent ductus arteriosus

- Kidneys- polycystic kidney, horseshoe kidney

- Male Genitals- enlarged penis, cryptorchidism

- Female Genitals- enlarged clitoris

- Hair- sparse, silvery-blonde scalp hair

- Cranial Nerve Paralysis, Moyamoya disease, Stroke, Intellectual disability

8p23.1 duplication syndrome is a rare genetic disorder caused by a duplication of a region from human chromosome 8. This duplication syndrome has an estimated prevalence of 1 in 64,000 births and is the reciprocal of the 8p23.1 deletion syndrome. The 8p23.1 duplication is associated with a variable phenotype including one or more of speech delay, developmental delay, mild dysmorphism, with prominent forehead and arched eyebrows, and congenital heart disease (CHD).

De Barsy syndrome is a rare autosomal recessive genetic disorder. Symptoms include cutis laxa (loose hanging skin) as well as other eye, musculoskeletal, and neurological abnormalities. It is usually progressive, manifesting side effects that can include clouded corneas, cataracts, short stature, dystonia, or progeria (premature aging).

It was first described in 1967 by De Barsy et al. and, as of 2011, there have been 27 cases reported worldwide. The genes that cause De Barsy syndrome have not been identified yet, although several studies have narrowed down the symptoms' cause. A study by Reversade et al. has shown that a mutation in PYCR1, the genetic sequence that codes for mitochondrial enzymes that break down proline, are prevalent in cases of autosomal recessive cutis laxa (ARCL), a condition very similar to De Barsy syndrome. A study by Leao-Teles et al. has shown that De Barsy syndrome may be related to mutations in ATP6V0A2 gene, known as ATP6V0A2-CDG by the new naming system.

Alternative names for De Barsy syndrome include corneal clouding-cutis laxa-mental retardation, cutis laxa-growth deficiency syndrome, De Barsy–Moens–Diercks syndrome, and progeroid syndrome of De Barsy.

Almost all children with Jacobsen syndrome have Intellectual disabilities, which ranges from mild to moderate depending upon the number of the deletion of genes from the chromosome. Children with intellectual disability take more time than normal to learn new things and acquire new skills. They have problems with assembling new information or adapting to novel situations and associating two events or things together.

Most kids with the syndrome have delayed development including delayed speech, motor disabilities, lack of coordination, which makes even simple activities like sitting, standing and walking difficult for these children. Most kids eventually start speaking but in cases with severe intellectual disability language use is highly restricted.

They have distinctive facial features like:

- Small head (microcephaly)

- Pointed forehead, (trigonocephaly)

- Small ears which are low-set

- Widely-spaced eyes (hypertelorism)

- Droopy eyelids (ptosis)

- Broad nasal bridge

- Abnormally thin upper lips

- Downturned corners of the mouth

- Excess skin covering in the inner corner of eyes (epicanthal folds)

Some children also suffer from behavioural problems like distractibility, hyperactivity, impaired communication and social skills which qualifies them for a diagnosis of ASD and ADHD.

Heart defects are very common in children with Jacobsen Syndrome. 88.5% of people with the disorder have Paris-Trousseau syndrome which is a bleeding disorder and causes a lifelong risk of abnormal bleeding and bruising due to dysfunction in the platelets.

Other symptoms may include eye problems, ear and sinus infections, hearing problems, bone deformities, growth hormone deficiency, gastrointestinal problems, kidney malfunctions etc.

Almost all patients with this syndrome have some degree of mental retardation and facial dysmorphism (round face, deep-set eyes, thin upper lip). Behavioural problems are common. Brachymetaphalangism (metacarpal or metatarsal shortening) is reported in ~50% of cases overall, but is typically not evident below the age of 2 years. There is striking phenotypic variability, and the size and extent of the deleted region cannot be used as accurate predictors of prognosis. Some patients have additional problems such as congenital heart disease and seizures.

Jacobsen Syndrome is a rare chromosomal disorder resulting from deletion of genes from chromosome 11 that includes band 11q24.1. It is a congenital disorder. Since the deletion takes place on the q arm of chromosome 11, it is also called 11q terminal deletion disorder. The deletion may range from 5 million to 16 million deleted DNA base pairs. The severity of symptoms depends on the number of deletions. The more deletions there are more severe the symptoms are likely to be. People with Jacobsen syndrome have serious intellectual disabilities, dysmorphic features, delayed development and a variety of physical problems including heart defects. Research shows that almost 88.5% of people with Jacobsen Syndrome have a bleeding disorder called Paris-Trousseau syndrome. [ Jacobsen Syndrome is catastrophic in 1 out of every 5 cases, since children usually die within the first 2 years of life due to heart complications.

The following is a list of symptoms that have been associated with Roberts syndrome:

- Bilateral Symmetric Tetraphocomelia- a birth defect in which the hands and feet are attached to shortened arms and legs

- Prenatal Growth Retardation

- Hypomelia (Hypoplasia)- the incomplete development of a tissue or organ; less drastic than aplasia, which is no development at all

- Oligodactyly- fewer than normal number of fingers or toes

- Thumb Aplasia- the absence of a thumb

- Syndactyly- condition in which two or more fingers (or toes) are joined together; the joining can involve the bones or just the skin between the fingers

- Clinodactyly- curving of the fifth finger (little finger) towards the fourth finger (ring finger) due to the underdevelopment of the middle bone in the fifth finger

- Elbow/Knee Flexion Contractures- an inability to fully straighten the arm or leg

- Cleft Lip- the presence of one or two vertical fissures in the upper lip; can be on one side (unilateral) or on both sides (bilateral)

- Cleft Palate- opening in the roof of the mouth

- Premaxillary Protrusion- upper part of the mouth sticks out farther than the lower part of the mouth

- Micrognathia- small chin

- Microbrachycephaly- smaller than normal head size

- Malar Hypoplasia- underdevelopment of the cheek bones

- Downslanting Palpebral Fissures- the outer corners of the eyes point downwards

- Ocular Hypertelorism- unusually wide-set eyes

- Exophthalmos- a protruding eyeball

- Corneal Clouding- clouding of the front-most part of the eye

- Hypoplastic Nasal Alae- narrowing of the nostrils that can decrease the width of the nasal base

- Beaked Nose- a nose with a prominent bridge that gives it the appearance of being curved

- Ear Malformations

- Intellectual disability

- Encephalocele (only in severe cases)- rare defect of the neural tube characterized by sac-like protrusions of the brain

Mortality is high among those severely affected by Roberts syndrome; however, mildly affected individuals may survive to adulthood

The syndrome is a rare clinical disorder.

- Physical

- Overgrowth

- Accelerated skeletal maturation

- Dysmorphic facial features

- Prominent eyes

- Bluish sclerae

- Coarse eyebrows

- Upturned nose

- Radiologic examination

- Accelerated osseous maturation

- Phalangeal abnormalities

- Tubular thinning of the long bones

- Skull abnormalities

- Mental

- Often associated with intellectual disability (of variable degree)

Bohring–Opitz syndrome (BOS) is a medical syndrome caused by a mutation in the ASXL1 gene. It is diagnosed by genetic testing and is characterised by characteristic craniofacial appearance, fixed contractures of the upper limbs, abnormal posture, feeding difficulties, intellectual disability, small size at birth, and failure to thrive. Some of these features are shared with other genetic syndromes.

Genetically, de novo truncating mutations in ASXL1 have been shown to account for approximately 50% of Bohring–Opitz syndrome cases.

The syndrome is extremely rare, with fewer than 80 known cases worldwide. The leading cause of death is respiratory infections. Children with BOS can have feeding difficulties, recurring respiratory infections, sleep apnea, developmental delay, failure to thrive, abnormal hair density and length, Wilm’s Tumors, brain abnormalities, silent aspiration, and other issues.

Approximately 100 cases have been described in the literature to date.

The facial features are characteristic and include

- Deep set eyes

- Strabismus

- Myopia

- Marked nasal root

- Broad and/or beaked nasal bridge

- Prominent Cupid's bow

- Everted lower lip

- Tented upper lip

- Large mouth

- Widely spaced teeth

- Wide and shallow palate

- Ears with thick and overfolded helix

Most have a smiling appearance.

Intellectual disability is severe. Language is absent or limited to only a few words. Stereotypic movements particularly of the arms, wrists and fingers is almost universal. Hypotonia is common (75%) as is an unsteady gait. All have delayed walking. Other features include a single (simian) palmar crease, long, slender fingers, flat feet and cryptorchidism (in males). Finger clubbing and the presence of fetal pads is common. Hyperventilation occurs in over half and is frequently followed by apnea and cyanosis. During these episodes aerophagia may occur. Constipation is common. Microcephaly and seizures may occur. Hypopigmented skin macules have occasionally been reported.