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What causes Down syndrome?
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 48 chromosomes, carrying the same genes and DNA.
In patients 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 plus two sex 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). 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 Down syndrome patients, it is 47+2. Phenotype describes the physical and functional capabilities of a patient. In patients with Down syndrome there is great variability in phenotype.