Down Syndrome: Types, Causes, and Symptoms

Down syndrome, also known as trisomy 21, is a genetic disorder that causes physical and cognitive delays in children as well as medical problems in adults. Most people with Down syndrome have distinctive facial features, intellectual disabilities, and health issues such as respiratory and hearing problems, along with other medical problems associated with the condition. There are three main types of Down syndrome: trisomy 21, translocation Down syndrome (trisomy 21 with different chromosomes), and mosaic Down syndrome (some cells have trisomy 21 while others do not). Most children with Down syndrome are born to parents who are both at least 40 years old.

Definition

Down syndrome is a chromosomal disorder caused when an individual has a full or partial extra copy of chromosome 21. This extra  genetic material changes  the course of development and result in  the characteristics related to Down syndrome. There are three types of Down syndrome, each caused by a different meiotic error. The most common type is trisomy 21, which occurs when each cell in the body has three copies of chromosome 21 instead of the usual two copies. Mosaic Down syndrome is caused by the presence of an extra chromosome 21 in only some of the body's cells. The third type, translocation Down syndrome, occurs when part of chromosome 21 breaks off during cell division and attaches to another chromosome. This type is relatively rare.

Mosaicism

Mosaicism happens  when a baby is born with an extra chromosome in some but not all of their cells. Babies  with mosaic Down syndrome  have fewer symptoms than those with trisomy 21.

Trisomy

This is also the  type of Down syndrome which is called as  trisomy 21, which occurs when three copies of chromosome 21 are present in the cells instead of the usual two copies. This form of Down syndrome is caused by a random error in cell division called nondisjunction. Nondisjunction happens   during  meiosis I or meiosis II.

Translocation

In this type of Down syndrome, new borns  have only an extra part of chromosome 21. There are 46 total chromosomes. One of them has an additional part  of chromosome 21 attached.

Symptoms

The three kinds  of Down syndrome are trisomy 21 (nondisjunction), translocation, and mosaicism. Trisomy 21 is the most common type, occurring in 95% of cases. Translocation occurs in about 4% of cases, while mosaicism is the least common, occurring in about 1% of cases. Down syndrome can cause a variety of symptoms, including physical and mental abnormalities. The severity of these signs and  symptoms changes  from man  to man. Common physical symptoms include a small head, low muscle tone, poor coordination, and short stature. Mental symptoms can include mild to moderate intellectual disability, delayed speech and language skills, and learning difficulties. In some cases, people with Down syndrome may also have problems with their heart, digestive system, hearing, or vision.

Causes

Down syndrome is caused by an error in cell division that results in an extra chromosome 21. This extra chromosome results in the physical and developmental characteristics associated with Down syndrome. There are three types of Down syndrome, each with its own cause: trisomy 21 (nondisjunction), translocation, and mosaicism.