Question

What is the source of extra chromosome 21?

Answer 1

Down syndrome is caused due to a genetic abnormality in which an extra chromosome number 21 is present, also called trisomy of chromosome 21. It leads to physical and mental disabilities of various kinds.

Complete answer:
As discussed above, down syndrome is a genetic abnormality resulting due to the presence of an extra copy of chromosome number 21, Thus, it is a chromosomal disorder.

Down syndrome (2n+1) occurs due to extra chromosome number 21, caused by abnormal cell division. During normal cell division, chromosomes are duplicated and each daughter cell receives a copy of each chromosome. Sometimes, these chromosomes are unable to separate properly and result in unequal division of chromosomes in each daughter cell. This condition is called non-disjunction of chromosomes.
Due to this non-disjunction of chromosomes, daughter cells receive abnormal number of chromosomes, i.e. one cell receiving one chromosome 21 while the other receiving an extra chromosome 21. In the majority of cases, the extra copy of chromosome 21 comes from the mother in the egg. In a small percentage (less than 5%) of cases, the extra copy of chromosome 21 comes from the father through the sperm. In the remaining cases, the error occurs after fertilization, as the embryo grows.

Many times a part of chromosome 21 can translocate to another part of another chromosome. This can also lead to Down syndrome as even though there is a normal number of chromosomes but the presence of an extra genetic material from chromosome 21.

Additional Information: Prominent signs and symptoms of Down ’s syndrome include:
- Short height and undersized growth
- Folding of skin above eyes
- Flattened nose and furrowed tongue
- Mental retardation
- Broad and short hands
- Heart deformities
- Short neck, small head and abnormal teeth
- Longer time in developing linguistic abilities
- Poor toning of muscle and excessive flexibility

Note: Down’s syndrome is named after the physician who first discovered it, Langdon Down. It affects physical as well as mental characteristics. People with Down’s syndrome are also susceptible to other disease conditions like Alzheimer’s and leukemia.