What Causes Down Syndrome
Table of Contents
[Down Syndrome](#Down Syndrome)
[Down Syndrome Causes](#Down Syndrome Causes)
[Down Syndrome Types](#Down Syndrome Types)
[Down Syndrome Symptoms](#Down Syndrome Symptoms)
[Frequently Asked Questions](#Frequently Asked Questions)
Down Syndrome
Down Syndrome
Definition - It is a genetic disorder, which is caused by the presence of an extra chromosome, known as Trisomy 21 or Chromosome 21. This leads to physical and mental disabilities, and is one of the leading causes of genetic disorders around the world.
It is named after the physician Langdon Down, who first observed this condition.
People with Down Syndrome are not only affected by physical attributes and mental retardation, but also by various diseases such as Leukemia and Alzheimer’s. Unfortunately, there is no cure for these diseases, but the quality of life can be improved by providing extra care and training the individual to perform daily essential activities. Early screening during pregnancy can help to diagnose Down Syndrome and reduce the risk of these diseases.
Karyotype of Down Syndrome (Trisomy of Chromosome 21)
Down Syndrome Causes
Down syndrome is a chromosomal disorder caused by aneuploidy of the autosome. This disorder is characterized by the presence of an extra chromosome 21 or part of the chromosome in all or some cells.
The occurrence of Down syndrome is 1:800 live births. The major risk factor is the age of the mother. Most of the trisomy cases occur in the mother having age more than 35.
Down syndrome is caused by nondisjunction, a phenomenon where the chromosomes are unable to separate during mitotic and meiotic cell division, resulting in cells with an unequal number of chromosomes.
Nondisjunction occurs when replicated chromosomes fail to segregate properly during anaphase, resulting in one daughter cell having one fewer chromosome and the other having one extra chromosome. This occurs when not all of the replicated chromosomes’ kinetochores are attached to microtubules from opposite poles during metaphase.
Down Syndrome is caused by the translocation of a part of chromosome 21 to another chromosome, resulting in an individual having an extra part of chromosome 21. The total number of chromosomes remains normal.
There is no genetic information missing in these individuals, however, the extra copy of genes on chromosome 21 can lead to abnormal physical and mental development.
Down syndrome is usually not inherited. However, Translocation Down syndrome can be passed on from parent to child. Balanced translocation in the parent may not cause any symptoms, but the unbalanced translocation can be passed to the child, causing Down syndrome.
The life span of an individual with Down syndrome is typically around 60 years, although this can vary depending on the number of health complications present.
Down Syndrome Types
Down syndrome is of three types:
- Trisomy 21
- Mosaicism
- Translocation
1. Trisomy of Chromosome 21: Trisomy 21, also known as Down Syndrome, is the most common type of Down Syndrome, accounting for 95% of all cases.
There are 47 chromosomes present instead of the normal 46, due to an extra chromosome 21.
The main cause of trisomy is Nondisjunction of chromosome 21 during meiosis at the time of gamete formation. An abnormal cell with trisomy of chromosome 21 is then fertilized, resulting in trisomy in all the cells of the foetus.
Mosaicism: This rare form of Down syndrome accounts for only 1% of all cases.
In Mosaic Down Syndrome, some cells contain the typical 46 chromosomes while other cells contain an extra chromosome (47 chromosomes). Symptoms of this condition may be less severe than other types of Down Syndrome.
Mosaicism occurs when nondisjunction occurs during mitotic division in the zygote after fertilization, resulting in some normal cells and some cells with trisomy of 21.
3. Translocation Down Syndrome: This type of Down Syndrome accounts for approximately 4% of all cases.
There is an extra part of chromosome 21 present, attached to a different chromosome, resulting in a total of 46 chromosomes, one of which is abnormal.
Chromosome 21’s long arm (q arm) is often translocated to another chromosome during the replication process, resulting in Translocation Down Syndrome. This syndrome involves a person having one chromosome 14, one chromosome 14/21 and a pair of normal chromosome 21. It is not linked to the mother’s age and may be passed down in families.
Down Syndrome Symptoms
Individuals with Down Syndrome are born with many abnormalities, including physical and mental disabilities, poor immunity, and a higher susceptibility to certain diseases. They reach developmental milestones at a later age than normal, and can be born with congenital heart defects, thyroid disease, sleep apnea, and gastrointestinal defects. They are particularly prone to diseases such as leukaemia and Alzheimer’s.
People with Down Syndrome can be easily identified by their physical attributes and facial features. As they develop, they may have distinct signs such as being of normal size at birth but having a shorter height than those of the same age as they grow.
The main symptoms of Down Syndrome are:
Short Stature and Stunted Growth
Fold of the skin above the eye, slanted eyes
Protruding furrowed tongue, flattened nose
Mental Retardation
Cardiac Deformities
Single Transverse Palmar Crease and Broad, Short Hands
Poor Muscle Tone and Excessive Flexibility
Small head, short neck, and abnormal teeth
Delay in Language Development
Cognitive impairment may range from mild to moderate
Down Syndrome Diagnosis
Children with Down syndrome can be diagnosed both prenatally and postnatally.
Due to their unique facial features, people with this condition can be easily identified. Unfortunately, if the condition is diagnosed after birth, there is no cure available, however, quality of life can be improved through training, education, and extra care.
If a foetus is found to have Down Syndrome during pregnancy, the pregnancy is usually terminated.
Amniocentesis is a technique used to diagnose Down syndrome. A needle is used to take out amniotic fluid, which contains cells with the same genetic content as the foetus. The karyotype of these cells is then studied in a lab to look for any chromosomal abnormalities. If an abnormality is found, the foetus is aborted.
Frequently Asked Questions
What are the Causes of Down Syndrome?
Down Syndrome is a genetic disorder caused by abnormal cell division, resulting in the presence of an extra chromosome 21, also known as Trisomy of Chromosome 21. This can be an entire extra chromosome 21, or just a partial extra chromosome present in all or some cells.
Characteristic Features of Down Syndrome
- Delayed physical development
- Low muscle tone
- Small stature
- Flat facial features
- Short neck
- Upward slanting eyes
- Poor coordination
- Speech and language delays
- Intellectual disability
The main characteristic of Down Syndrome is Intellectual Disability. Other physical characteristics may include stunted growth, a protruding tongue, a flattened nose, a short neck, a small head, poor muscle tone, and more.
