Genomic resource centre

Genes and human disease

Genes and chromosomal diseases

Down Syndrome

Down Syndrome is a type of mental retardation caused by extra genetic material in chromosome 21. This can be due to a process called nondisjunction, in which genetic materials fail to separate during a crucial part of the formation of gametes, resulting in an extra chromosome (called trisomy 21). The cause of nondisjunction is not known, although it correlates with a woman’s age reference. The additional material present influences development and results in the state known as Down Syndrome.

Some physical characteristics of Down Syndrome in infants are decreased muscle tone, a flat face, eyes slanting up, irregular shaped ears, ability to extend joints beyond the usual, large space between the big toe and its neighbouring toe, large tongue relative to the mouth, etc.

Down Syndrome patients can also be afflicted with other conditions, including heart disease, Alzheimer’s disease, and leukaemia.


The estimated incidence of Down Syndrome is between 1 in 1,000 to 1 in 1,100 live births worldwide. Each year approximately 3,000 to 5,000 children are born with this chromosome disorder and it is believed there are about 250,000 families in the United States of America who are affected by Down Syndrome.

Sixty to 80 percent of children with Down Syndrome have hearing deficits. Forty to 45 percent of children with Down Syndrome have congenital heart disease. Intestinal abnormalities also occur at a higher frequency in children with Down Syndrome. Children with Down Syndrome often have more eye problems than other children who do not have this chromosome disorder. Another concern relates to nutritional aspects. Some children with Down Syndrome, in particular those with severe heart disease often fail to thrive in infancy. On the other hand, obesity is often noted during adolescence and early adulthood. These conditions can be prevented by providing appropriate nutritional counselling and anticipatory dietary guidance. Thyroid dysfunction are more common in children with Down Syndrome than in normal children. Skeletal problems have also been noted at a higher frequency in children with Down Syndrome. Other important medical aspects in Down Syndrome, including immunologic concerns, leukaemia, Alzheimer disease, seizure disorders, sleep apnoea and skin disorders, may require the attention of specialists in their respective fields.

Diagnosis / prognosis:

The prognosis of Down Syndrome is variable depending upon the possible complications like heart defects, susceptibility to infections and development of leukaemia. In the early 1900’s, Down Syndrome patients were expected to live less than 10 years. Now, about 80% of adult Down Syndrome patients reach their 50th birthday and beyond. The screening methods commonly used in the case of Down Syndrome are expanded Alpha Fetoprotein (AFP) , Nuchal Translucency Screening (NT) , amniocentesis, Chorionic Villus Sampling (CVS) and ultrasound. These diagnostic tests inform the parents of the mental and physical needs of the child and enabling them to be prepared for the challenges ahead. The quality of life of Down Syndrome patients can be improved by meeting their health care needs, which include: regular check-ups with health professionals to monitor mental and physical growth and to provide timely intervention be it physiotherapy, counselling or special education. Down Syndrome patients can achieve optimal quality of life through parental care and support, medical guidance and community based support systems like special schools. This facilitates their participation in mainstream society and the fulfilment of their personal potential.

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