Down Syndrome: Causes, Types, Symptoms and Diagnosis

Down syndrome or Trisomy 21 is a genetic disorder where individuals are born with an extra or partial copy of the chromosome. A normal human being has 46 chromosomes (23 pairs), but those affected with Down syndrome have 47 chromosomes. 

• This can affect the development of the brain and body including the physical features.
• The disease was first observed by Langdon Down, therefore it is named Down Syndrome.

Figure: Trisomy of Chromosome 21

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What is Down Syndrome?

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Humans have a pair of 23 chromosomes, this means in every cell there are a total of 46 chromosomes present. Down syndrome is a disorder wherein a person has an extra copy of the 21^st chromosome resulting in 47 chromosomes in total.

• This change in number can lead to many problems like poor brain and body development.
• Individuals with the disease have flat faces, short height, huge tongue, and poor social skills.
• As per recent reports, Down syndrome affects 25,000 babies every year.

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Types of Down Syndrome

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Down Syndrome is caused by a mutation in genes. Based on these variations, the disorder is divided into three types:

1. Trisomy 21
2. Translocation
3. Mosaicism

Trisomy 21

Down syndrome is trisomy 21 as there are three copies of the 21^st chromosome instead of two. This is caused by non-disjunction, which is defined as the inability of the chromosomes to separate during cell division or meiosis. 

• When the 21^st chromosome fails to segregate at the time of cell division it causes the additional chromosome to replicate in every cell of the body.
• It is one of the most common types and about 95% of the cases are known. 

Translocation

In translocation, an extra chromosome is not present, but there is a presence of an extra part of the chromosome. This means that a portion of the 21^st chromosome breaks off and translocates to another chromosome (usually the 14^th chromosome) and attaches to it.

• An individual with this syndrome has a normal chromosome, a 14 chromosome, and a 14/21 chromosome.
• It is one of the rarest types and about 4% of the cases are known. 

Mosaicism

Mosaicism is caused by the nondisjunction of chromosome 21 when the zygote undergoes mitosis cell division after fertilization. This disease shows very few visible characteristics as compared with the other types.

• In this type, a few cells contain 46 chromosomes (normal) while a few are 47 chromosomes (abnormal).
• This is also one of the rarest types of Down syndrome as only 1% of the total cases are known.

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Causes of Down Syndrome

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The major cause of Down syndrome is the abnormal division of cells. In Down syndrome, excess division of cell occurs leading to an unequal number of chromosomes. This unequal number of chromosomes is called nondisjunction. There are several risks of Down syndrome such as:

• Women older than 35 years of age have greater chances of giving birth to a child with Down Syndrome.
• Both parents can be the carriers of the translocation Down Syndrome.
• Already having a child with the same disorder may increase the probability of having another child with trisomy 21

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Symptoms of Down Syndrome

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Every individual with Down Syndrome is born with various intellectual and developmental problems. In some, the abnormalities are mild while in others they are severe. A person who has this syndrome is more likely to be susceptible to various other diseases such as mental retardation, leukemia, Alzheimer’s, immune disorder, etc. It is easy to identify individuals with Down syndrome by their physical features. The most common features or symptoms of Down syndrome are as follows:

• Small and short neck 
• Presence of a large gap between the first and second toe.
• Slanting eyes
• Facing problems while speaking 
• Intellectual disabilities 
• Mental retardation 
• Poor muscle development 
• Presence of irregular teeth 
• Flattened nose and protruding tongue 

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Diagnosis of Down Syndrome

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Several diagnosis methods are available for the identification of Down syndrome. We will be discussing the two major diagnosis methods that are used to identify and determine the risk of down syndrome.

Prenatal Screening Test

This test is performed to determine the risk of having a child with Down Syndrome.

• Screening tests may include a blood test where the doctors look for the increased levels of hCG hormone in the blood.
• Other tests include cell-free DNA tests and ultrasound. 

Diagnosis during Pregnancy

This test confirms whether the child has Down Syndrome. This test is usually a risk to the parent and the fetus. This disorder can be diagnosed by the amniocentesis technique.

• In this technique, amniotic fluid is taken with a needle's help, and the karyotype is done which helps to determine chromosomal disorders.
• If the karyotype shows abnormality, it means that the fetus is abnormal.

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Treatment of Down Syndrome 

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There is no treatment available for Down syndrome. However, there are therapies available like behavioral counseling, special educational services, etc.

• Physical and speech therapies are provided to children at an early age, where they are taught how to sit, walk, and speak.
• Everyday skills like learning how to use a computer, and how to dress and eat are taught by occupational therapists.
• According to US law, free education is provided to children up to 21 years or when the complete high school.

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Things to Remember

• The presence of an extra chromosome in the human body is referred to as Down syndrome.
• Dementia is one of the most common risks for people suffering from this syndrome.
• The lifespan of individuals with Down syndrome is 60 years.
• The separation of chromosomes before replication in anaphase is called nondisjunction.
• Blood tests, screening tests, and ultrasound are the methods of diagnosis for this disease.

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Previous Years Questions

1. Which of the following is an example of pleiotropy ?…..[ CUCET 2011]
2. Who proposed the chromosomal theory of inheritance ?….[CUCET 2010]
3. The children of a father with type 'O' blood and mother with type….[COMEDK UGET 2012]
4. Epicanthus skin fold above the eyes and transverse palmer….[COMEDK UGET 2007]
5. Distance between the genes and percentage of….[COMEDK UGET 2015]
6. Which one of the following is a sex-linked disease….​[CUCET 2010]
7. The science that deals with the application of principles….[CUCET 2010]
8. A 10 years old patient is found to have the following….[COMEDK UGET 2012]
9. Which of the following genes shows the heterozygous...[BHU UET 2008]
10. Which chromosomal constitution refers to Jacob's syndrome….[BHU UET 2012]
11. Which of the following is an example of intergenic gene….
12. Which of the following is not considered as a mutagen…
13. Haemophilia is an example of….
14. In order to lessen the suffering of phenylketonurics...[AMUEEE 2014]
15. In Mendel's experiments with garden pea, round seed shape..[JKCET 2015]
16. Genotypic and phenotypic ratio in monohybrid cross remains..[JKCET 2011]