Know About The Types of Down Syndrome

Down syndrome is a common chromosomal abnormality that affects an individual in his/her fetal stage of development. It is termed trisomy 21 since an extra copy of chromosome 21 is present. It is responsible for causing retarded physical as well as mental growth in a baby, leading to disabilities that may be lifelong. The disease can shorten life expectancy, however, with advancements in treatment modalities quality of affected lives has been improved. Cultural and institutional support as well support from family and peers also help to overcome the challenges related to the condition.

Depending upon the pattern of inheritance of the 21st chromosome, Down syndrome may be classified into the following three types -

• Trisomy 21 Down syndrome - It is the most common form of Down syndrome, comprising 95 per cent of the cases. This condition is caused due to an error in cell division, known as ‘nondisjunction’ and characterized by the presence of an extra copy of chromosome 21. The mechanism involves the failed separation of one of the chromosomes of the 21st pair, either in sperm or egg. It results in an extra chromosome in the embryo's DNA instead of getting one chromosome each from the mother and father.

• Translocation Down syndrome - It accounts for only 3 to 4 percent of the cases of Down syndrome. An extra full or partial copy of chromosome number 21 is present in this condition. It is just like trisomy 21 but the only difference is that the extra full or partial 21st chromosome is attached to any of the chromosomes of 23 pairs. That means the person having Down syndrome has a normal number of chromosomes which is 46, out of which anyone chromosome is bound with an extra 21st chromosome.

• Mosaic Down syndrome - This is the rarest form of Down syndrome which accounts for only one to two per cent of cases. In this condition, an extra copy of the 21st chromosome is present in some of the cells but not in all. The number, as well as severity of the symptoms, is less in individuals who are suffering from this type of Down syndrome as compared to the other two types.

There are some basic features in individuals suffering from Down syndrome. Those may include :

• Delayed growth and development

• Eyes, which are almond-shaped

• Degraded muscle tone

• Small hands and feet with shorter extremities

• Short height

• A flattened nasal bridge

• Short neck with shorter ears