Down Syndrome or Down’s Syndrome also known as Trisomy 21 is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. The presence of this extra genetic material causes the developmental changes and physical features of Down Syndrome. The parents of the affected individual are usually genetically normal. The probability increases from less than 0.1% in 20 year old mothers to 3% in those of age 45. The extra chromosome occurs due to the non-disjunction of chromosomes that is the failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division. Down syndrome can be identified during pregnancy by prenatal screening followed by diagnostic testing or after birth by direct observation and genetic testing.

Down Syndrome is one of the most common chromosome abnormalities in human. It occurs in about 1 in 1000 babies born each year. In 2015, Down syndrome was present in 5.4 million individuals globally and resulted in 27000 deaths, down from 43000 deaths in 1990. It is named after British Doctor John Langdon Down who fully described this syndrome. Down syndrome varies in severity among individuals, causing lifelong intellectual disability and developmental delays. It’s the most common genetic chromosomal disorder and cause of learning disabilities in children. It also commonly causes other medical abnormalities, including heart and gastrointestinal disorders.


Each person with Down syndrome is an individual- intellectual and developmental problems may be mild, moderate or severe. Some people are healthy while others have significant health problems such as serious heart defects. Children and adults with Down syndrome have distinct facial features. Some of the common features include:-

  • flattened face.
  • small head.
  • short neck.
  • protruding tongue.
  • upward slanting eye lids(palpebral fissures).
  • unusually shaped or small ears.
  • poor muscle tone.
  • broad, short hands with a single crease in the palm.
  • short height.
  • relatively short fingers and small hands and feet.


Human cells normally contain 23 pairs of chromosomes. One chromosome in each pair comes from the father and the other from mother. Down syndrome results when abnormal cell division involving chromosome 21 occurs. Any one of the three genetic variations discussed below can cause Down syndrome:-

  1. Trisomy 21- about 95% of the time, Down syndrome is caused by trisomy 21. The person has three copies of chromosome 21, instead of the usual two copies, in all cells. This is caused by abnormal cell division during the development of the sperm cell or the egg cell.
  2. Mosaic Down syndrome- very rare. The person has only some cells with an extra copy of chromosome 21. This mosaic of normal and abnormal cells is caused by abnormal cell division after fertilization.
  3. Translocation Down syndrome- occurs when a portion of chromosome 21 becomes attached (translocated) onto another chromosome, before or at conception. These children have two copies of chromosome 21 as usual but they also have additional genetic material from chromosome 21 attached to another chromosome.


  • Heart defects- about half of the children with down syndrome are born with some type of congenital heart defect.
  • Gastrointestinal defects(GID)- includes abnormalities of the intestine, esophagus, trachea and anus.
  • Immune disorders- people with Down syndrome are at increased risk of developing autoimmune disorders.
  • Obesity
  • Dementia
  • Spinal disorders

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