Most people have 23 pairs of chromosomes in most or all of their cells for a total of 46 chromosomes in all. These chromosomes include DNA and other material that provide a blueprint for “building” a person.

Some people have trisomy conditions—those related to having an extra chromosome in most or all of their cells, for a total of 47 chromosomes in all.

An extra chromosome can cause a variety of health problems ranging from mild intellectual and developmental disability (IDD), to severe physical problems.

About Trisomy Conditions

Trisomy occurs when a person has a full or partial extra chromosome in most or all of his or her cells—47 chromosomes total.

The specific health issues of a trisomy condition and how severe those issues are depend on:

• Which chromosome: An extra copy of some chromosomes is lethal; for example, an embryo with three copies of chromosome 1 will not develop.

• Whether there is a partial or complete extra chromosome: If cells include only a partial extra chromosome, symptoms are usually milder than if cells have a complete extra chromosome.

• How many cells have the extra chromosome or partial chromosome: If the extra chromosome is in only a few cells, the symptoms are usually milder than if the extra chromosome is in all or most of cells.

Health conditions and problems associated with trisomy include physical abnormalities, such as extra fingers or toes; physiological issues, such as irregular heartbeat patterns; and problems related to intellectual and developmental functioning.

Most of the time, trisomy conditions are not passed from one generation to the next, but result from a random error that occurs during cell division very early on in development.

Trisomy can occur with any chromosome, but the most well-known syndromes are:

• Trisomy 21, more commonly known as Down syndrome, occurs in 1 in 691 live births in the United States each year, according to the Centers for Disease Control and Prevention. People with Down syndrome usually have mild-to-moderate intellectual and developmental disability (IDD), heart abnormalities, and are at risk for hearing and vision loss and a number of other health conditions.

• Trisomy 18, also called Edwards syndrome, occurs in about 1 in 5,000 live births each year in the United States. Infants with Trisomy 18 often have severe IDD, as well as serious heart problems and other life-threatening issues that make them less likely to live past their first birthday.

• Trisomy 13, also called Patau syndrome, occurs in about 1 in 10,000 to 16,000 live births each year worldwide. Trisomy 13 is associated with more severe IDD and multiple physical problems, including serious heart problems.

Trisomy 2 mosaicism is a rare chromosome disorder characterized by having an extra copy of chromosome 2 in a proportion, but not all, of a person's cells. In infants born with Trisomy 2 mosaicism, severity as well as signs and symptoms vary widely. Features of Trisomy 2 mosaicism may include intrauterine growth restriction (IUGR), any of various birth defects, distinctive facial features, growth delay, developmental delays, and intellectual disabilities. The severity and specific symptoms present generally depend on the level of mosaicism (the proportion of cells affected) and the location or type of affected cells in the body. Trisomy 2 mosaicism is not inherited. It is caused by a random error in cell division during early development of the embryo.

When Trisomy 2 mosaicism is detected during early pregnancy with chorionic villus sampling (CVS), the affected cells may be confined only to the placenta, and not present in the fetus. Amniocentesis may be recommended to confirm the results of the CVS, and monitoring may occur to detect intrauterine growth restriction, low amniotic fluid level (oligohydramnios), or other complications including stillbirth.

Mosaic trisomy 8 is a chromosomal abnormality that can affect many parts of the body. In individuals with Mosaic trisomy 8, some of the body's cells have three copies of chromosome 8 (trisomy), while other cells have the usual two copies of this chromosome. The signs and symptoms vary, but may include distinctive facial features; intellectual disability; and joint, kidney, cardiac, and skeletal abnormalities. Males are more frequently affected than females. Most cases are not inherited; it often occurs sporadically as a random event during the formation of the reproductive cells (egg and sperm). Diagnosis may be based on analysis of chromosomes via genetic testing, such as karyotype.

Mosaic trisomy 9 is a chromosomal abnormality that can affect may parts of the body. In people affected by this condition, some of the body's cells have three copies of chromosome 9 (trisomy), while other cells have the usual two copies of this chromosome. The signs and symptoms vary but may include mild to severe intellectual disability, developmental delay, growth problems (both before and after birth), congenital heart defects, and/or abnormalities of the craniofacial (skull and face) region. Most cases are not inherited; it often occurs sporadically as a random event during the formation of the reproductive cells (egg and sperm) or as the fertilized egg divides.

Trisomy 13 is a type of chromosome disorder characterized by having 3 copies of chromosome 13 in cells of the body, instead of the usual 2 copies. In some people, only a portion of cells contains the extra chromosome 13 (called mosaic Trisomy 13), whereas other cells contain the normal chromosome pair. Trisomy 13 causes severe intellectual disability and many physical abnormalities, such as congenital heart defects; brain or spinal cord abnormalities; very small or poorly developed eyes (microphthalmia); extra fingers or toes; cleft lip with or without cleft palate; and weak muscle tone (hypotonia). Most cases are not inherited and result from a random error during the formation of eggs or sperm in healthy parents. Trisomy 13 is diagnosed based on the symptoms, clinical exam, and confirmed by the results of a chromosome test.

Mosaic trisomy 14 is a rare chromosomal disorder in which there are 3 copies (trisomy) of chromosome 14 in some cells of the body, while other cells have the usual two copies. The extent and severity of features in affected individuals can vary. Signs and symptoms that have been most commonly reported include intrauterine growth restriction; failure to to thrive; developmental delay; intellectual disability; distinctive facial characteristics; structural malformations of the heart; and other physical abnormalities. This condition is most often caused by an error in cell division in the egg or sperm cell before conception, or in fetal cells after fertilization.

Trisomy 17 mosaicism is a chromosomal abnormality in which there are three copies of chromosome 17 in some cells of the body, rather than the usual two copies. It is often incorrectly called trisomy 17 (also referred to as full trisomy 17), which is when three copies of chromosome 17 are present in all cells of the body. Few cases of Trisomy 17 mosaicism have been described, most having been detected during pregnancy through a test called amniocentesis. Because the proportion and location of cells with trisomy 17 differs from case to case, the presence and severity of signs and symptoms may vary significantly from person to person.

Trisomy 18 is a chromosome disorder characterized by having 3 copies of chromosome 18 instead of the usual 2 copies. Signs and symptoms include severe intellectual disability; low birth weight; a small, abnormally shaped head; a small jaw and mouth; clenched fists with overlapping fingers; congenital heart defects; and various abnormalities of other organs. Most cases are not inherited and occur sporadically (by chance).

Chromosome 20 trisomy, (also called trisomy 20) is a condition in which a fetus or individual has an extra full or partial copy of chromosome 20 in some or all of of his/her cells. The presence of an extra copy of only part of chromosome 20 is called partial trisomy 20; and an extra copy of chromosome 20 in only some of a person's cells is called mosaic trisomy 20. Mosaic trisomy 20 is the most common type of Chromosome 20 trisomy and is one of the more common chromosomal abnormalities found during prenatal diagnostic testing. However, features that have been reported include spinal abnormalities (including spinal stenosis, vertebral fusion, and kyphosis), hypotonia (decreased muscle tone), lifelong constipation, sloped shoulders, and significant learning disabilities despite normal intelligence. Trisomy 20 usually results from an error that occurs when an egg or sperm cell develops (before fertilization); mosaic trisomy 20 usually results from errors in cell division soon after fertilization.

Down syndrome is a chromosomal condition that is associated with intellectual disability, a characteristic facial appearance, and weak muscle tone (hypotonia) in infancy. All affected individuals experience cognitive delays, but the intellectual disability is usually mild to moderate.

People with Down syndrome often have a characteristic facial appearance that includes a flattened appearance to the face, outside corners of the eyes that point upward (upslanting palpebral fissures), small ears, a short neck, and a tongue that tends to stick out of the mouth. Affected individuals may have a variety of birth defects. Many people with Down syndrome have small hands and feet and a single crease across the palms of the hands. About half of all affected children are born with a heart defect. Digestive abnormalities, such as a blockage of the intestine, are less common.

Individuals with Down syndrome have an increased risk of developing several medical conditions. These include gastroesophageal reflux, which is a backflow of acidic stomach contents into the esophagus, and celiac disease, which is an intolerance of a wheat protein called gluten. About 15 percent of people with Down syndrome have an underactive thyroid gland (hypothyroidism). The thyroid gland is a butterfly-shaped organ in the lower neck that produces hormones. Individuals with Down syndrome also have an increased risk of hearing and vision problems. Additionally, a small percentage of children with Down syndrome develop cancer of blood-forming cells (leukemia).

Delayed development and behavioral problems are often reported in children with Down syndrome. Affected individuals can have growth problems and their speech and language develop later and more slowly than in children without Down syndrome. Additionally, speech may be difficult to understand in individuals with Down syndrome. Behavioral issues can include attention problems, obsessive/compulsive behavior, and stubbornness or tantrums. A small percentage of people with Down syndrome are also diagnosed with developmental conditions called autism spectrum disorders, which affect communication and social interaction.

People with Down syndrome often experience a gradual decline in thinking ability (cognition) as they age, usually starting around age 50. Down syndrome is also associated with an increased risk of developing Alzheimer's disease, a brain disorder that results in a gradual loss of memory, judgment, and ability to function. Approximately half of adults with Down syndrome develop Alzheimer's disease. Although Alzheimer's disease is usually a disorder that occurs in older adults, people with Down syndrome commonly develop this condition earlier, in their fifties or sixties.

Trisomy 22 is a chromosome disorder in which an extra (third) copy of chromosome 22 is present in every cell of the body where there should normally only be two copies. Common features include an underdeveloped midface (midface hypoplasia) with flat/broad nasal bridge, malformed ears with pits or tags, cleft palate, hypertelorism (wide-spaced eyes), microcephaly and other cranial abnormalities, congenital heart disease, genital abnormalities, and intrauterine growth restriction (IUGR).

Mosaic trisomy 22 is a chromosome disorder in which chromosome 22 is present three times, instead of the usual two times, in some cells of the body. The range and severity of the disorder can vary widely. Some of the features that have been associated with this condition include growth delays, cognitive deficiencies, unequal development of the two sides of the body (hemidystrophy), webbing of the neck, abnormal deviation of the elbows when extended (cubitus valgus), multiple pigmented moles or birthmarks, distinctive malformations of the head and face, and other physical abnormalities. A number of cases of children with Mosaic trisomy 22 and normal growth and development have also been described.

Trisomy X, also called triple X syndrome or 47,XXX, is characterized by the presence of an additional X chromosome in each of a female's cells. Although females with this condition may be taller than average, this chromosomal change typically causes no unusual physical features. Most females with trisomy X have normal sexual development and are able to conceive children.

Trisomy X is associated with an increased risk of learning disabilities and delayed development of speech and language skills. Delayed development of motor skills (such as sitting and walking), weak muscle tone (hypotonia), and behavioral and emotional difficulties are also possible, but these characteristics vary widely. Seizures or kidney abnormalities occur in about 10 percent of affected females.

Klinefelter syndrome, also called 47,XXY, is a chromosomal condition that affects development in people who are assigned male at birth. The signs and symptoms of Klinefelter syndrome vary. In some cases, the features are so mild that the condition is not diagnosed until puberty or adulthood. Researchers believe that up to 65 percent of people with Klinefelter syndrome are never diagnosed.

Individuals with Klinefelter syndrome typically have small testes that produce a reduced amount of testosterone (primary testicular insufficiency). Testosterone is the hormone that directs male sexual development before birth and during puberty. A small percentage of affected individuals are born with undescended testes (cryptorchidism). Without treatment, the shortage of testosterone can lead to delayed or incomplete puberty, breast enlargement (gynecomastia), decreased muscle mass, decreased bone density, a reduced amount of facial and body hair, and fatigue. Klinefelter syndrome can make it difficult for people with this condition to have biological children (a condition called infertility), but up to half of people with Klinefelter syndrome may be able to have children using assisted reproductive technologies..

The other physical changes associated with Klinefelter syndrome are usually subtle. Most commonly, affected individuals are taller than average and 2 to 3 inches taller than would be expected for their family. Other features can include curved pinky fingers (fifth finger clinodactyly), flat feet (pes planus), and, less commonly, abnormal fusion of certain bones in the forearm (radioulnar synostosis).

Children with Klinefelter syndrome may have low muscle tone (hypotonia), difficulty coordinating movements, and mild delays of certain developmental skills, such as rolling over or walking. Affected children have an increased risk of mild delays in speech and language development. People with Klinefelter syndrome tend to have better receptive language skills (the ability to understand speech) than expressive language skills (vocabulary and the production of speech) and may have difficulty communicating and expressing themselves. Affected individuals have an increased risk for learning disabilities, most commonly problems with reading (dyslexia) and written expression. People with Klinefelter syndrome very rarely have intellectual disabilities.

Individuals with Klinefelter syndrome may have have anxiety, depression, impaired social skills, or behavioral differences, such as emotional immaturity during childhood or difficulty with frustration. Affected individuals also have an increased risk for attention-deficit/hyperactivity disorder (ADHD), though they tend to have problems with attention and distractability rather than hyperactivity. People with Klinefelter syndrome are more likely than those without Klinefelter syndrome to have autism spectrum disorder, which is a developmental disorder that affects communication and social interaction.

People with Klinefelter syndrome have an increased risk of developing metabolic syndrome, which is a group of conditions that include high blood glucose levels during prolonged periods without food (fasting), high blood pressure (hypertension), increased belly fat, and high levels of fats (lipids) such as cholesterol and triglycerides in the blood. Compared with unaffected people, adults with Klinefelter syndrome also have an increased risk of developing involuntary trembling (tremors) in their arms or hands, breast cancer (if gynecomastia develops), thinning and weakening of the bones (osteoporosis), and autoimmune disorders such as systemic lupus erythematosus and rheumatoid arthritis. Autoimmune disorders are a large group of conditions that occur when the immune system attacks the body's own tissues and organs.