Affects one in 800 newborns
Most cases of Down syndrome result from trisomy 21, which means each cell in the body has three copies of chromosome 21 instead of the usual two copies.
Individuals with Down syndrome are characterised by typical facial features such as an oval-shaped face and eyes that slant upwards. All will have some degree of learning difficulty, delayed speech development and a delay in motor development. Approximately 50% of babies with Down syndrome are born with a heart defect and are also at risk of developing other medical conditions.
Although women of any age can have a child with Down syndrome, the chance of having a child with this condition increases with maternal age.
Affects one in 5,000 newborns
Most cases of Edwards syndrome result from trisomy 18, which means each cell in the body has three copies of chromosome 18 instead of the usual two copies.
The extra genetic material disrupts the normal course of development, causing slow growth before birth and a low birth weight. Heart defects and abnormalities of other organs may also develop before birth.
Many individuals with trisomy 18 die before birth or within their first month. Only 5-10% of affected children live past their first year, and often have severe intellectual disability.
Affects one in 16,000 newborns
Most cases of Patau syndrome result from trisomy 13, which means each cell in the body has three copies of chromosome 13 instead of the usual two copies.
Many fetuses don’t survive until full-term and are stillborn or spontaneously abort.
Features include slow growth before birth, low birth weight, heart defects, organ malformation, brain and central nervous system abnormalities and craniofacial abnormalities.
People typically have two sex chromosomes in each cell: females have two X chromosomes, and males have one X and one Y chromosome. These types of aneuploidies occur when there is a change in the normal structure of sex chromosomes.
Affects one in 1,000 females
Triple X syndrome results from an extra copy of the X chromosome in each of a female's cells.
Females with this condition may be taller than average, but it typically causes no unusual physical features. Most females have normal sexual development and are able to conceive children. There is an associated risk of learning disabilities and delayed development of speech, language and motor skills. Seizures or kidney abnormalities occur in about 10% of affected females.
Affects one in 2,500 females
Most often, Turner syndrome results when one X chromosome is normal and the other X chromosome is missing or altered.
The most common feature of Turner syndrome is short stature, which becomes evident by about age 5. An early loss of ovarian function is also very common. Up to half of affected females are born with a heart defect, complications of which can be life threatening. However, most girls and women with the condition have normal intelligence.
Affects one in 500 to 1,000 males
Most often, Klinefelter syndrome is the result of one extra copy of the X chromosome in each cell, making it three X chromosomes in total.
The condition interferes with sexual development, such as reduced levels of testosterone, which can lead to delayed, or incomplete puberty, breast enlargement, reduced facial and body hair, and infertility. It also causes abnormal functioning of the testes.
Affects one in 1,000 males
Jacob’s syndrome is the result of one extra copy of the Y chromosome in each cell. Although males with this condition may be taller than average, this chromosomal change typically causes no unusual physical features. Most have normal sexual development and are able to father children.
There is an increased risk of learning disabilities and delayed development of speech, language and motor skills. A small percentage of males are diagnosed with autistic spectrum disorders, which are developmental conditions that affect communication and social interaction.
These occur when there are alterations in the normal chromosomal structure, such as deletions of part of a chromosome. The resulting abnormalities are determined by the size of the deleted portion.
Affects one in 5,000-10,000 newborns
This condition results when a piece of chromosome 1 is missing, and typically causes severe intellectual disability. Most affected individuals have limited or no speech, as well as behavior problems. Most have structural abnormalities of the brain resulting in seizures. There may also be weak muscle tone and swallowing difficulties.
Physical characteristics include a small head that is also unusually short and wide in proportion to its size, facial features that include deep-set eyes with straight eyebrows; a sunken appearance of the middle of the face; a pointed chin; and abnormally shaped ears. There might also be vision or hearing problems, and abnormalities of the skeleton, heart, gastrointestinal system and other organs.
Affects one in 20,000-50,000 newborns
This condition results when a piece of chromosome 5 is missing. Affected infants have a high-pitched cry that sounds like a cat, hence the name Cri-du-chat (cat's cry).
It causes intellectual disability and delayed development, small head size, low birth weight, and weak muscle tone in infancy. Affected individuals also have distinctive facial features, including widely set eyes, low-set ears, a small jaw and a rounded face. Some infants with cri-du-chat syndrome are born with a heart defect.
As they grow, people with cri-du-chat usually have difficulty walking and talking. They may have behavioural problems and severe intellectual disability.
This rare condition results when a piece of chromosome 2 is missing. It affects the motor neurons, which are the specialised nerve cells in the brain and spinal cord that control the movement of muscles. The disorder usually causes severe mental retardation and behaviour problems.