What chromosome causes Down syndrome?
Down syndrome is caused by the presence of an extra chromosome.
Normally, the human body has 23 pairs of chromosomes (called autosomes) and two sex chromosomes (allosomes). At conception, a new cell is formed that receives one copy of each chromosome from the sperm and one copy from the egg. The new cell divides and multiplies to form an embryo and ultimately a fetus and new human. Each cell contains the exact same genetic material as the original 46 chromosomes, carrying the same genes and DNA.
Does an extra chromosome cause Down syndrome?
In people with Down syndrome, an error occurs in the coming together of chromosome 21. The extra genetic material is responsible for the developmental abnormalities that occur. Instead of 46 chromosomes, there are 47.
The most common error in chromosome replication is trisomy 21, where the new cell gets three copies of chromosome 21, instead of two. This accounts for about 95% of those patients with Down syndrome. Translocation describes a less common event where an extra piece of chromosome 21 gets attached to another chromosome, again delivering more genetic material to the new cell than is needed.
Mosaic Down syndrome occurs when there is a combination of cells with the normal number of chromosomes (46+2) mixed together with those containing a third chromosome 21 (47+2). Note: +2 is the plus two sex chromosomes. The cells with normal chromosomes can moderate the effect of the trisomy 21 cells and modify the effect on the patient's physical and mental development.
Genotype is the term used to describe the genetic makeup of a person and for most people with Down syndrome, it is 47+2. Phenotype describes the physical and functional capabilities of a patient. In individuals with Down syndrome, there is great variability in phenotype.
What is Down syndrome?
Down syndrome is a genetic disorder and the most common autosomal chromosome abnormality in humans, in which extra genetic material from chromosome 21 is transferred to a newly formed embryo. The extra genetic material causes changes in the development of the embryo and fetus resulting in physical and mental abnormalities. Each person with Down syndrome is unique and there can be great variability in the severity of symptoms.
The most common type of Down syndrome is trisomy 21 and Mosaic Down syndrome is the rare type of Down syndrome.