Chromosome Number in Down Syndrome – Information at a Glance
|Ploidy||3 copies of chromosome number 21|
|Occurrence||Full or a partial extra copy of chromosome 21|
What is Down Syndrome?
Also referred to as trisomy 21, Down syndrome is a genetic disorder that is a common genetic birth defect affecting close to 1 in 800 infants.
A normal sperm cell and normal egg cell starts with 46 chromosomes. Then these sperm and egg cells divide in half, and then the sperm and egg cells have 23 chromosomes each. When such sperms with 23 chromosomes fertilize an egg with 23 chromosomes, the offspring will have a complete set of 46 chromosomes; half are obtained from the mother and half from the father.
At some occasions, an error takes place when 46 chromosomes are being reduced to half. A sperm cell or egg may retain both copies of chromosome number 21 rather than 1 copy only. If such a sperm or egg is fertilized, the offspring shall have 3 copies of chromosome number 21, referred to as Trisomy 21.
At times, the extra number 21 chromosome or a section of it is in association with another chromosome in the sperm or egg, which may lead to translocation Down syndrome. This form of Down syndrome is only when that can be inherited from a parent.
Mosaic trisomy 21 is a rare form of it, and it takes place when an error has taken place in the process of cell division once fertilization has occurred. Those individuals with this syndrome possess normal cells and a few cells with extra chromosome number 21.
What is the reason for the occurrence of Down Syndrome?
Number of Chromosomes in Down Syndrome
Typically, human cells comprise 23 pairs of chromosomes, and one pair comes from each of the biological parents. The condition of Down syndrome takes place when abnormal cell division involving chromosome 21 takes place, hence there is an extra chromosome. This abnormalcy in the cell division leads to an extra full or partial chromosome 21. This genetic material which is extra, is involved in contributing to the distinct features and developmental conditions of Down syndrome. Down syndrome can be caused by any one of these three genetic variations.
- Mosaic Down syndrome – It is a rare form of Down syndrome wherein the individual has only a few cells with an extra copy of chromosome 21. This type of mosaic of abnormal and normal cells is as a result of the abnormal division of cells after fertilization.
- Trisomy 21 – The majority of the Down syndrome condition is as a result of Trisomy 21, wherein the individual has three copies of chromosome 21 rather than two usual copies in all the cells. This is a result of abnormal cell division at the time of development of the egg and sperm cell.
- Translocation Down syndrome – The condition of Down syndrome can take place when a part of the chromosome 21 gets translocated or attached to another chromosome at the time of conception or before it. Such individuals usually have two copies of chromosome 21 in addition to having genetic matter from chromosome 21 being associated with another chromosome.
This was a brief on the Down syndrome chromosome number.
Principles of Inheritance and Variation Class 12 Biology | NEET 2022
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Frequently Asked Questions Down syndrome
What is the number of chromosomes in Down’s syndrome?
Down’s syndrome or trisomy 21 is a genetic condition caused due to the presence of an additional chromosome. Normally, a baby inherits 23 chromosomes from each parent summing up to 46 chromosomes. However, babies with Down syndrome possess an extra chromosome at position 21, hence three chromosomes instead of the normal 2 for a pair are seen. The medical term to having an extra chromosome is called trisomy.
When is Down’s syndrome caused?
The genetic condition, Down’s syndrome is caused when there is an abnormal cell division which leads to an additional partial or full copy of chromosome 21.