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Down’s Syndrome: Characteristics, Causes, Types, Diagnosis, Practice Problems and FAQs

Down’s Syndrome: Characteristics, Causes, Types, Diagnosis, Practice Problems and FAQs

Chromosomes are the thread-like structures present in the nucleus of a cell which carries the hereditary information from one generation to the next generation. Every species possesses a characteristic number of chromosomes in their cell. When irregularities occur at the time of cell division, alteration in the number or structure of chromosomes happens. These alterations can lead to various chromosomal disorders. Examples include Down’s Syndrome, Klinefelter’s Syndrome etc.

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                  Fig: Down’s Syndrome

Do you know 1 in around every 800 babies is affected by Down’s Syndrome, making it the most common chromosomal disorder. You might have seen some kids with this disorder. Now, you must be wondering what is a chromosomal disorder?. Well, a chromosomal disorder results when chromosomes or parts of chromosomes are changed, showing an abnormality in the total chromosome number. But what is it exactly that causes this disorder so frequently? To find out the answer to this question we need to understand Down’s syndrome first. In this article, we will discuss Down’s Syndrome in depth.

Table of contents

  • Syndrome
  • Karyotype
  • Down’s Syndrome
  • Characteristic features of Down’s Syndrome
  • Cause of Down’s Syndrome
  • Types of Down’s Syndrome
  • Diagnosis of Down’s Syndrome
  • Practice Problems
  • FAQs

Syndrome

Each human cell generally has a total of 46 chromosomes or 23 pairs of chromosomes. Mother (maternal copy) and father (paternal copy) both contribute one chromosome to form a homologous pair. The presence of a hereditary disorder may arise if there are fewer or more chromosomes present than the usual 46 chromosomes.

A syndrome is a class of medical conditions distinguished by a specific set of signs and symptoms. When this set of symptoms is seen at the chromosomal level, characterised by loss or gain of chromosomes, it results in a chromosomal abnormality, called a chromosomal disorder.

Karyotype

Karyotype represents the complete set of chromosomes (at metaphase stage), in any cell of a species or an individual organism, that are arranged according to length, centromere location and other traits.


                                                     Fig: Human karyotype

Down’s Syndrome

The genetic disorder known as Down’s syndrome is caused due to the presence of an extra copy of chromosome 21 inside body cells. It is caused by trisomy of chromosome number 21. The Greek words ‘tri’ means three and ‘soma’ means body. This was first described by John Langdon Down in 1866. It is an autosomal trisomy condition.


                           Fig- Karyotype of a person affected by Down’s syndrome

Characteristic features of Down’s Syndrome

The following are the common characteristic features of Down’s Syndrome:

  • Flattening of the nasal bridge.
  • Prominent forehead.
  • Epicanthal folds that give the eyes an Asian appearance.
  • Protruding tongue.
  • Small ears.
  • Habitually opened mouth.
  • Malformed heart.
  • Broad palm with palm crease.
  • Susceptible to respiratory problems.
  • Shorter life span.
  • Short stature.
  • Projected lower lip.
  • Large protruding tongue.
  • Underdeveloped gonads and genitalia.
  • Abnormal pelvis.


                       Fig. Physical features of a baby affected with Down’s syndrome

Common learning and psychological issues associated with Down’s Syndrome

Those affected with Down’s Syndrome typically display mild to moderate intellectual impairments. They also experience difficulties in learning and this results in delayed development. Moreover, coordination and fine motor skills may also be hindered. Common learning and psychological issues of Down’s Syndrome include the following:

  • Delays in speech and language development.
  • Attention problems.
  • Sleep difficulties.
  • Stubbornness and tantrums.
  • Delays in cognition.
  • Delayed toilet training.

Causes of Down’s Syndrome

A unique type of cell division called meiosis occurs for the production of gametes, the egg and sperm cells. The fusion of egg and sperm results in the formation of zygote which multiplies to form the embryo. Strangely, meiosis can go wrong, and the sperm or egg will receive an incorrect number of chromosomes. This condition can normally occur in the anaphase and can lead to unequal distribution of chromosomes in the daughter cells. It will lead to genetic diseases. The technical term for this condition is nondisjunction.


                        Fig: Nondisjunction

This indicates that a pair of chromosomes in the parent cell failed to split equally into separate daughter cells, such as sperm or eggs. A chromosomal imbalance at the beginning of pregnancy almost always results in miscarriage. The child is born with a chromosomal condition if the pregnancy does go on. The most prevalent of these syndromes, Down's syndrome and is caused by an extra copy of chromosome 21.


                      Fig: The nondisjunction in gametes during meiosis

Types of Down’s Syndrome

Down’s Syndrome is observed in three major types such as trisomy 21 (nondisjunction), mosaicism, and translocation.

Trisomy 21

Nondisjunction, which results in an embryo having three copies of chromosome 21 instead of the normal two, is the most common cause of Down’s Syndrome. Trisomy is the most common type of Down’s Syndrome. It accounts for 95% of Down’s Syndrome cases. The total number of chromosomes present is 47 instead of the normal 46 chromosomes here. The main cause of trisomy is the nondisjunction of chromosome number 21 during meiosis at the time of gamete formation. This abnormal cell with trisomy of chromosome 21 is fertilised giving rise to trisomy in all the cells of the foetus.

Mosaicism

The mosaic Down’s Syndrome or mosaicism occurs when there is a mixture of two types of cells, some containing the usual 46 chromosomes and some containing 47 chromosomes. The cells which contain the 47 chromosomes have three copies of chromosome 21. It is the least frequent type of Down’s Syndrome. Only 1% of all cases of Down’s Syndrome are caused by it. Symptoms may be less prominent in mosaicism. This condition occurs when nondisjunction occurs during mitotic division in the zygote after fertilisation.

Translocation

The total number of chromosomes in the cells stays at 46 in translocation, which accounts for around 4% of cases of Down’s Syndrome. But, an extra full or partial copy of chromosome 21 gets linked to another chromosome, most often chromosome 14. The traits of Down’s Syndrome are brought on by the extra whole or partial chromosome 21. So a person with translocation Down’s Syndrome contains one chromosome 14, one chromosome 14/21 and a pair of normal chromosome 21. Its occurrence is not related to the mother’s age and it may run in families.

Diagnosis of Down’s Syndrome

Children with Down’s Syndrome can be diagnosed before or after birth. Due to the presence of special physical characteristics they can be easily identifiable. There is no cure for this condition if the disease is identified after birth. In this case the quality of life can be improved by special education, proper training, and extra care. Down’s Syndrome can be diagnosed during pregnancy and if the foetus is found to be affected, then the pregnancy is terminated.

It is diagnosed by the amniocentesis technique. In this technique the amniotic fluid is taken out with the help of a needle and the karyotype of the dividing cells are done in the lab. The karyotype is studied for any chromosomal abnormality. If the karyotype is found abnormal, then the foetus is aborted.


                        GIF: Amniocentesis

Practice Problems

1. People with flattened nose bridges and significant defects in cognitive development suffer from which chromosomal disorder?

a. Down’s Syndrome
b. Turner’s Syndrome
c. Klinefelter’s Syndrome
d. Cri-du-chat Syndrome

Solution: Down’s syndrome is a chromosomal abnormality that results from a trisomy of chromosome number 21. People affected with this syndrome generally have a flat nose and a protruding tongue. They have difficulties in speaking, learning and there is overall delayed cognitive development. Hence, the correct option is A.


                        Fig. Physical features of a baby affected with Down’s syndrome

2. Down’s Syndrome may be diagnosed through ________________.

a. amniocentesis
b. urine test
c. blood test
d. chromosomal karyotype

Solution: Diagnostic tests such as amniocentesis and ultrasound are prevalently used to diagnose the presence of Down’s Syndrome. In amniocentesis the amniotic fluid is taken out with the help of a needle and the karyotype of the dividing cells are done in the lab. The karyotype is studied for any chromosomal abnormality. If the karyotype is found abnormal, then the foetus is aborted. This diagnostic test accurately diagnoses Down’s syndrome and other chromosomal problems. Hence, the correct option is A.


                              GIF: Amniocentesis

3. Which of the following types of chromosomal abnormalities are most commonly associated with Down’s Syndrome?

a. Translocation
b. Trisomy 21
c. Deletion
d. Mosaicism

Solution: In most cases of Down’s syndrome, there is an extra chromosome 21 i.e., trisomy of 21. This type is seen in 95% of the cases. Only about 1% are genetically mosaic, that is, some of their cells have abnormalities in chromosome 21 while others do not, resulting in generally milder symptoms. Only about 3 - 5% of people with Down’s Syndrome have this syndrome due to a chromosome structural modification. They have a translocation of all or part of chromosome 21 to chromosome 14. Hence, the correct option is B.

4. What is syndrome?
Answer:
A syndrome is a class of medical conditions distinguished by a specific set of signs and symptoms.

FAQs

1. What is the primary cause of Down’s Syndrome?
Answer:
The chromosomal disorder Down's Syndrome is caused due to the presence of an additional copy of chromosome 21 in cells of the body due to a random error in cell division. This condition can normally occur in the anaphase and can lead to unequal distribution of chromosomes in the daughter cells. It will lead to genetic diseases. The technical term for this condition is nondisjunction. The exact cause of this nondisjunction is unknown.

2. Can people with Down’s Syndrome have normal children?
Answer:
Yes, a woman affected with Down’s Syndrome can have normal children with a probability of 50% if her partner is normal. If both parents have this condition, the chances of having a child born with Down’s Syndrome increase significantly.

3. Is there a cure for Down’s Syndrome?
Answer:
Currently, there is no cure for Down’s Syndrome. To help persons with Down’s Syndrome realise their full potential, there are numerous physical and developmental therapies available. The quality of life can be improved by special education, proper training, and extra care.

4. What impact does a person with Down’s Syndrome have on society?
Answer:
Despite the improvements in the general understanding and management of Down’s Syndrome, this disorder still carries some stigma around it in society. People with Down’s Syndrome are becoming more and more accepted in society and community organisations, including workplaces, academic institutions, healthcare systems, recreational and social activities.

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