Down Syndrome Overview (cont.)

Medical Author:
Medical Editor:

What causes Down syndrome?

Comment on this

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.

What are the signs and symptoms of Down syndrome?

The baby with Down syndrome has a hallmark appearance. However, every aspect of the appearance does not need to be present as the phenotype can be markedly different for each patient.

Commonly, there is a small head and short neck, a flat face, and upward slanting eyes. Ears are flat and positioned lower than “normal.” The tongue protrudes and seems to be too large for the mouth. Hands tend to be wide, with short fingers and there is just a single flexion crease in the palm. Joints tend to be more flexible and muscles may lack tone.

The patient may have growth retardation and though as a baby may be normal size, will not grow as tall. Average height for an adult male with Down syndrome is 5 ft 1 in and for a female it is 4 ft 9 in. Bowleggedness is common. Obesity occurs with aging.

There is decreased mental function and the IQ may range from mild disability (50 to 70) to moderate (35 to 50). For patients with Mosaic Down Syndrome, the IQ can be 10 to 30 points higher. There can be language development delay both from hearing impairment and speech delay. Gross motor skills like crawling and walking can be slow to mature and fine motor skills may take time to develop.

Medically Reviewed by a Doctor on 10/31/2013

Patient Comments

Viewers share their comments

Down Syndrome - Symptoms Question: What Down syndrome symptoms does your child exhibit?
Down Syndrome - Tests Question: Did you have special screening tests for Down syndrome during your pregnancy?
Down Syndrome - Causes Question: What caused your Down syndrome?
Down Syndrome - Experience Question: Please describe your experience with Down syndrome.