Down's syndrome

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Down's syndome is disorder caused by having an extra chromosome on chromosome 21 (referred to as trisomy 21), giving a total of 47 chromosomes instead of the normal 46 in humans.This extra chromosome copy disrupts the normal course of brain and body development, thus affecting both physical development and intellectual ability. Although individuals with Down's syndrome behave and look similar, their intellectual capability varies from mild to moderately low. As sufferers age (often about the age of 50), they may often experience a progressive decline in cognitive function, which can result in an increased risk of Alzheimer's, a neurodegenerative disease that causes dementia: a gradual loss of memory, motor skills, and judgement. 



There are three main types of Down's syndrome.

Trisomy 21

An error during gamete formation results in a failure of the chromosomal pair on chromosome 21 to separately properly, which results in an embryo with three copies of chromosome 21. This is known as non-disjunction. Most cases of Down's syndrome are a result of a random chromosomal defect during the formation of reproductive cells, especially egg cells.  As the embryo develops, the extra chromosome is replicated in every cell of the body. This is the most common form, and accounts for about 95% of cases. 

Mosaic trisomy 21

Mosaic refers to mixed or a combination. Most of the cells in the body have an extra chromosome, whereas some has normal pair of chromosomes. Like trisomy 21, this isn't inherited; instead it is caused by random events that occur during cell division in early foetal development. Consquently, unlike trisomy and translocation 21, symptoms can differ according to the number of abnormal chromosomes that the sufferer has. This is the least common form and accounts for about 1% of cases. 

Translocation trisomy 21

Part of chromosome 21 is in the cell, which becomes attached to another chromosome during gametic cells formation or during early foetal development. This is transferred to other cells as it divides. In some cases, parents unaffected by Down's syndrome may have translocation trisomy 21, which can be inherited by their offspring. 


To date, the cause of the extra of partial chromosome is still unknown, Increasing maternal age (especially pregnant women aged 35 or over) is a contributory factor towards a higher risk of a baby having Down's syndrome with trisomy 21 or mosaicism. There is no conclusive scientific research to suggest that environmental factors or parental activities before or during preganancy are the causes. 



Young children are more susceptible to developmental delay. This may be a consequence of poor muscle tone, which impacts the ability to stand, balance, and sit normally, so they may reach these milestones slower than other children,

Intelletual and behavioural

Screening and diagnosis

Before a clinical diagnosis is made, prenatal screening tests are carried out to predict the probability of the foetus having Down's syndrome. Conversely, diagnostic tests are more accurate in determining whether or not the foetus will have Down's syndrome. 


Prenatal diagnosis

Chorionic villus sampling - obtain sample of material from the placenta and test for chromosal abnormalities. 

Amniocentisis - examines amniotic fluid (fluid sac surrounding the baby), and test it for protein levels

Percutaneous umbilical blood sampling  - examines blood from the umbilical cord

Diagnosis at birth

Non-invasive prenatal testing

Karyotope analysis of newborn - obtain a blood sample from newborn and observe the cells. 


Related conditions and disorders


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