Which describes the condition in which a child is born with an extra 21st chromosome?

• Down’s syndrome was first described in 1866 by John Langdon Down.
• Down’s syndrome (also known as Down syndrome and trisomy 21) occurs in one in every 1,000 live births but accounts for around 2 per cent of all spontaneous abortions.
• The risk of having a child with Down’s syndrome increases significantly with maternal age to 1 in 50 if the mother is over 45 years of age.

Genetics

• Down’s syndrome is the result of a chromosomal abnormality in chromosome 21.
• Down’s syndrome is the result of an additional copy of all, or a specific part, of chromosome 21. This results in three partial or complete copies of the chromosome, also known as trisomy 21.
• The additional chromosome 21 can occur by:
• Non-disjunction (‘not coming apart’):
  • This is when a pair of chromosomes or sister chromatids fail to separate properly during cell division (mitosis or meiosis).
  • Both mitosis and meiosis involve the ordered distribution of chromosomes to form daughter cells.
  • If non-disjunction occurs, it can result in individuals having an abnormal number of chromosomes (for example, three copies of chromosome 21) in their cells.
  • Non-disjunction is responsible for 95 per cent of Down’s syndrome cases.
• Translocation:
  • One copy of chromosome 21 becomes attached to another chromosome and is inherited as a passenger on that other chromosome.

The karyotype from an individual with Down’s syndrome (trisomy 21). Image credit: Wessex Reg. Genetics Centre, Wellcome Images

Biology

• Researchers believe that having extra copies of genes on chromosome 21 disrupts the course of normal human development, causing the characteristic features of Down’s syndrome.
• However, scientists are still trying to understand the exact mechanisms behind the disorder and research is ongoing.

Symptoms

• Individuals tend to have mild to moderate intellectual disability.
• They may find sitting, standing, walking, talking and reading very difficult during development.
• Their growth may be limited.
• People with trisomy 21 tend to have characteristic facial features including slanting eyes, small ears, an open mouth with protruding tongue and a small nose.

People with Down’s syndrome tend to have mild to moderate intellectual disability.
Image credit: Shutterstock

• Short-sightedness (myopia) and deafness are common in people with Down’s syndrome.
• Some children may also experience symptoms of autism or attention deficit hyperactivity disorder (ADHD).
• Congenital (present from birth) heart defects are present in 50 per cent of children with Down’s syndrome.
• They may also have digestive problems resulting in symptoms such as constipation.
• They have an increased risk of infection.
• Life expectancy can be as high as 40-50 years and there is no effect on fertility.
• Older people with Down’s syndrome are at increased risk of developing dementia and Alzheimer’s disease.

People with Down’s syndrome can live into their 40s and 50s.

Image credit: Shutterstock

Diagnosis

• Down’s syndrome is generally diagnosed at birth by the facial appearance of the baby, muscle weakness, sleepiness and the presence of extra folds of skin around the neck.
• Blood tests are carried out to confirm the presence of the extra copy of chromosome 21.
• All pregnant women, regardless of their age, are offered screening tests for Down’s syndrome during the first 12 weeks of their pregnancy.
• Screening tests for Down’s syndrome usually take the form of:
  • a blood test: detects the presence of proteins and hormones associated with pregnancy, usually pregnancy-associated plasma protein A (PAPP-A) and beta human chorionic gonadotrophic (beta-hCG). If the baby has Down’s syndrome, the levels of these substances can be affected.
  • an ultrasound scan: done between 11 weeks and 2 days and 14 weeks and 1 day of pregnancy. The scan measures the fluid collection under the skin at the back of the baby’s neck, the ‘nuchal translucency’. All babies have a collection of fluid here but babies with Down’s syndrome tend to have more fluid in this area.
• The results from these two tests are then combined with the mother’s age to work out their individual risk of having a baby with Down’s syndrome.
• These test results do not give a definitive diagnosis that a baby has Down’s syndrome but they do give an idea of what the likelihood will be.
• Cell-free foetal DNA analysis, a test which involves examining baby DNA found in the mother’s blood, is becoming a common method for testing for Down’s syndrome. It is more often carried out in mother’s with a higher risk of having a baby with Down’s syndrome or if a risk is identified by one of the other tests.
• Two other tests can give a definitive diagnosis of Down’s syndrome. They are associated with a small risk of miscarriage and counselling will be offered if the results are positive.
• These tests are:
  • amniocentesis: this involves taking a sample of amniotic fluid through a needle inserted into the mother’s stomach. The fluid is then tested for chromosomal abnormalities (in the case of Down’s syndrome this will be an extra chromosome 21). Result in three days.
  • chorionic villus sampling (CVS): this involves taking a sample of the placenta through a needle inserted through the stomach and into the womb, or a tube passed through the vagina and cervix into the womb. This sample is then screened for an extra chromosome 21. Result in three days.

Treatment

• People with Down’s syndrome can lead relatively normal and successful lives.
• Children with Down’s syndrome often need special educational support and tend to reach developmental milestones later than unaffected children.
• People with Down’s syndrome tend to have regular medical checks to identify heart, hearing, visual or skeletal problems as soon as possible.
• Speech and physical therapy can assist children with Down’s syndrome to reach their full potential.
• Mouse models are currently being developed in the lab to allow scientists to advance their understanding of the specific genes involved and improve medical treatments for patients with Down’s syndrome.

People with Down’s syndrome can lead relatively normal and successful lives.

Image credit: Shutterstock

This page was last updated on 2021-07-21

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